I know he's a bike mechanic, but have you tried videoing him while he's riding? It can be very revealing, and people who think they have an ideal set up could be shifting in their saddle, too far forward or back in the saddle, be extending too much at the knee or ankle, or indeed be too low on the saddle requiring other compensations.

Cyclists also like to keep the knees tucked in to reduce drag, but should ideally have the knees driving down over the pedals - if he is internally rotating it will have a direct impact on the knee and thus the ITB.

Get him to bring in a roller and video him from the back and side, he might be surprised.