I had a trainer that would roll hard to one direction. found that one of the wings was twisted. I was able to correct it by twisting it in the opposite direction and using the heat gun to shrink the covering. But if it is happening with up elevator check and make sure that when he made the joiner for the elevators he didnt put in any differental. One elevator higher then the other. Here is a link to an easy to make differental gauge.

Differental Guage