You/he have several options,
1/. it could be that a high spot has been left & actually a bit more needs removing to stop the action rocking like a seesaw.

2/. it could be that the mounting bolt is bottoming out before it fully tightens so measure the depth, it might just need either a washer or a shorter bolt.

3/. Shim the fit with thin rubber sheet, as above a sliced open inner tube can be perfect.

4/. Properly "bed the action" with epoxy, cavedweller1959 on youtube gives good clear details, but remember several coats of release agent