Ok, some advice.
See how difficult the songs really are. Talk it over with the other musicians to see what they can handle. Then, have a discussion with the MOM and let them know the deal from the gate.
Forget about skill level; that really doesn't matter if one doesn't have the time to put into learning new songs (unless they're near professional caliber, IMHO).
Have a discussion and see what can be done. That's all I've got.
