I built one about 6 years ago and recently sold it ( HUGE MISTAKE).
Its easy, to avoid nicking lines, you can store co2 inside. Later on, he can decide on relocating if he wants. Will need more frequent bottle exchanges if you keep the co2 inside, because for the physics laws of gas pressure. But anyway, I found a thread or link on the internet and followed that.
I bought a new Roper dent and scratch fridge with upper freezer. I removed all the shelving. I put a 1/8" thick piece of wood on the floor, and kept a metal support bar at the back of the fridge compartment. The wood covered about 2/3 of the floor, and the back 1/3 was raised, but the metal support beam forms a ledge on which the kegs rested. It easily fit 6 kegs with the co2 tank outside. I had a brass manifold with 6 outlets, mini shutoffs, and and single input. Try to get a manifold that has check valves. If you have the cash, they also have manifolds with mini regulators on each output, to but the mini regs are fairly expensive. Scout out a fridge first and have dimensions calculated for the kegs and bring a tape measure to see if they'll fit. Look at the back floor of the fridge, it's often raised.
After that, if you want, you can remove the molded door panel with a Dremel tool, or if your really lucky, a screwdriver. Be careful not to damage the door seal however. If you want, you can keep the door panel in place though. It just makes the taps have a little play as the plastic compresses when you tighten the beer nuts.
Now you need to drill out the holes for the beer shanks. Get 6 inch stainless steel shanks if you can afford, otherwise chrome plated. You'll need one beer nut for each one. You also need tail pieces. I prefer the SS flare nut type, but for starters, get the hose barb style. 3/16 " bev tubing is what you are going to want, with appropriate sized hose barb tail pieces. Next are the faucets. You want the perlick types with forward seal. Trust me on this. Stainless steel. Buy the covers that slip on to keep out the fruit flies. Buy a faucet brush. You need a drip tray. Buy the cheapest one you can, that is long enough to cover drips from several taps. Tell hubby he can figure out how to mount it himself, because the counter top styles are cheapest, but have no mounting holes. Then you are going to need the keg connectors. There are ball lock and pin lock. Look at his kegs, and buy the type of gas and beer connectors to match, one for each faucet you're installing. You need neoprene beer washers one for each faucet. You need a regulator for the co2 tank if you don't have one already, and a tank and kegs if you don't already have. Last thing you need is gas line.
You'll probably need a trip to hardware store at some point. You'll need worm style hose clamps or oetiker style hose clamps. Get 4 per faucet plus several extras.
You'll need a drill and a hole saw the proper size for your beer shanks. I forget the correct size. You need a Dremel tool or hack saw (optional) if you want to remove the shelves on the door. You need a screwdriver, and a sharp utility knife. You also need a nice drill bit set if you want to drill the hole for gas line.
You need some small sheet metal screws. I like the hex head self tapping 3/8" long type, for mounting the manifold.
Using the screws mount the manifold up near the top of the fridge with outlets pointing UP, and inlet pointing toward the front of the fridge ideally. And try to mount it on the side of the fridge (inside) on which you would like the CO2 to sit- plan so that you can drill the fridge on that side to rest the co2 outside in the future if you want, without having to relocated the manifold. Leave a good 6"-8" from top of fridge inside chamber to top of manifold to easily mount the gas out lines. They point up so as to be out of the way. I like the rigid reinforced type gas tube. You need to cut the tubing and mount he gas disconnects with clamps and also clamp to manifold. Plumb in the co2 regulator the same way. See below if you're drilling the fridge side wall.
Next measure out the spacing of the faucets and shank holes on front door of the fridge. Be sure to account for the height of the taps and the freezer door so opening the freezer doesn't activate any faucets! Also be sure they are spaced close enough so the drip tray will cover them all. Drill the fridge front door with the hole saw and slide the shanks through. The threaded side will face inside, the cupped part the outside of the fridge. If you didn't remove the door shelves, jest tighten the retaining nuts snugly, and don't forget to use the trim ring on the outside. These should come with the shanks. Don't attach the faucets yet, you'll need a faucet wrench too by the way, but can use other types of wrenches in a pinch. Just be careful you don't scratch the shanks though.
Ok, next step. Cut the bev tubing to the right length, this is the tricky part because you won't know the right amount of resistance you want. In general, I used 10-11 feet of 3/16 tubing and had good results. Measure this for each keg. If you're feeling generous, get a big box of bev tubing so he can created custom lengths for different beer styles. Once you have the tubing cut you can proceed. Put a tail piece and beer nut on each shank being sure to use the neoprene washer in the beer nut, and tighten. Attach tubing and tighten worm clamp. Connect opposite end to beer out connector with a clamp.
Install faucets with faucet wrench, and leave the drip tray on huge floor for now. You could always use some corner braces or something form the hardware store to mount this to the from of the fridge with a little ingenuity. Be sure you can fit a glass on the tray below the faucets before mounting if you choose to do this.
Connect co2 tanks ( don't forget the washer if your regulator uses one). Dial pressure to 10-11 psi. Prepare a spray bottle with 50:50 dish soap to water. Spray all connections on the gas side to test for leaks. Fix any leaks, clean up soap. Get a mop bucket and a empty clean keg. Fill keg with hot soapy water. Connect keg to each tap one at a time, flush the lines with with soapy water. Collect soapy water in mop bucket from each tap. Rinse thoroughly with cold water in similar fashion. Now you can pour beer!
Drilling hole for co2. If you opt to do this, here are some tips. I bought some pipe nipples about 3-4" long, some o rings that slipped around the outside of each, and a drill bit big enough so that I could use the nipple as a conduit through the side of the fridge wall, to avoid sharp edges cutting the main gas feed line. I drilled two such holes so I could dispense beer and run a second line from a dual regulator setup so I could force carb a keg at a different pressure if I wanted, or to serve a nitro beer with another cylinder....
Anyway, I got coolant line during my drilling. Here is what to do to avoid a coolant line. Measure a 3x3 square on inside plastic lining of fridge. Use Dremel tool to barely cut out the plastic without getting too deep into insulation behind. Remove and save. Using a knife, or some other implement, dissect out the foam insulation to reveal the coolant lines that usually are on outer metal skin of the fridge. Then using your drill, drill out the holes with coolant lines in plain sight.match up the hole for the piece of plastic inner lining you saved and drill so you can use the pipe nipple conduit later. Tape the plastic piece back into place, and fill the hole with foam insulation and let harden. Go back and re-drill or dissect the foam and install the nipple / conduit (s). Use O rings to hold in place.
Other tips. If you want you can use permanent oetiker style clamps instead of worm clamps, which can erode the line over time. You can use flare nut style connections for tailpieces and keg connectors, which I beneficial if you want to serve commercial kegs from time to time, in switching between fittings. I much prefer these to barb type connections. Get 6" shanks. These help keep the beer in the faucet cold from the metal mass, with less foamy pours if the fridge is in a hot garage. Try to find and use all stainless steel parts instead of chrome plated. The chrome does not last and then you're drinking chrome and then machined brass with your beer. If you leave the shelves on the door inside the fridge, the taps will swivel somewhat. Don't over tighten the nuts trying to prevent this, but try to keep them all evenly tightened. Appearance and aesthetic issue only. Don't forget to accurately measure the hole spacing for the taps to prevent freeze door problems and drip tray issues. You may need a sample tap handle. Dry erase magnetic boards or tap boards would be a nice touch to label what's on Tap.
Good luck.
TD