You will need a double ended shank they are commonly used in jockey boxes, sometimes called a jockey box wall coupling. You can screw the standard beer nut, already on the line, to one side of the shank, on the other side you need a 1/4" tailpeice and beer nut(with washer). Then hook up a few feet of 3/16 beer line to the 1/4" tailpeice and the corny disconnect on the other.
This will cost you at least $20-25 per line( beer and gas) it's much cheaper to cut the line and add flare barbs to the hose. These cost $2 each and then buy him new flare tailpeices so he can hook the sanky back up these fitting are about $5 each. So either spend $50 or $14 to do the same thing with the only difference is cutting off a small peice of line.