This is very common and is simply because the program or part levels (or even both) are set too high. If he reduces these levels the distortion should disappear. The program level is probably the most likely of the two levels so try this one first. The control is in the amp envelope section. Once set you can resave the problem program with it's new level.
This is because doubling the number of notes played doubles the voice amp level. If you play an 8 note chord, the output will increase by 9 dBr compared to if you played 1 note. This can be enough to tip the voice amp into distortion.