First off, planes don't fly crazy if they really are straight and true.

Someone besides your buddy needs to check the plane for warps, thrust offset and tail angle. Maybe he is measuring from the wrong spot or doesn't know how to check.

The fin could be off. Lay a yardstick along one side and see if it runs down the center of the fuselage. You can stang in front of a plane and slowly kneel down, looking back and forth at the wings and usually spot a warp. Stand above and look down on the engine.

This answer is far out, but it happened to a buddy of mine. He hadn't had a computer radio long and I helped him set up flaperons and landing spoilers. He sold that plane and built another, using the same location in memory. Here's where the problem came in-he didn't do a RESET prior to starting the new program. This ought to be in big red letters, as far as I am concerned.

Here's the result. On the previous plane, he was using the mix switch for the landing spoilers. Naturally, he left it off. His new plane flew fine until one day he accidently bumped the mix switch. When he throttled to idle, the radio put the spoilers up. Right aileron-aileron channel up. Left aileron-flap channel up. Except, he was now using a Y-connector!. Right aileron up, left aileron down!! Throttle to idle, right roll. HELP. We got the plane down and fixed the program, but if your buddy has a computer radio, check it, there may be something weird in the program.