You may be able to use a 1/2" NPT Tap to clean up the threads in the coupling which is what your friend is suggesting. Stainless steel can be tough to work with and especially so if it has hardened some from the welding heat. It's at least worth a try.
Here's a link to the pipe tap so you can see what one looks like: http://www.ridgid.com/Tools/Pipe-Taps
You might have to go to a plumbing supply to get one. Home Depot probably won't have one and I'm not sure that Lowe's would either. Most local hardware stores do stock them, so you might try there first.
The taps should be used with a tap handle, but you might be better off using Vice Grips or a large adjustable wrench due to the thread being on the inside and more difficult to reach.
Start the tap carefully and when you are confident that the tap is chasing the threads and not cross threading, crank it in about 1/4 -1/2 of a turn. Back it out a little and advance another 1/4 turn. Use a lot of lubricating oil. Just about any oil will work for this, even some automotive motor oil. So, keep repeating this advance some and backing out a little then advancing more and so on. The coupling threads are tapered and so is the tap, so it will become increasing more difficult to proceed as you get deeper into the coupling. I would try to clean up at least five or six threads into the coupling if possible. This should allow you to thread the tube in at least three full turns and maybe a little more. If you have an unused coupling, you can thread the tap into it to see how it works and how far in it will go. You will probably have to use some fairly strong force to turn the tap. It might be a good idea to take the tap and the kettle back to the guy that did the welding. He will probably have some experience with taps and can help you with fixing the problem.
Sometimes people try to avoid what you are dealing with by putting a plug into the coupling when welding it. The problem with that technique is that often it will be very difficult to remove the plug after the welding has been completed. The threads on the plug and coupling can lock up almost as firmly as if they had been welded and it can be a major biotch to remove the plug.