Think I've also heard about some sites banning them. Myth or not wont matter if that's the case.
My advise to him would be to got them, and the unlikely event that a specific site don't allow them - just spend a tenner on their ammo. I do the opposite. I have some bio BBs, but only use them on non-airsoft-locations and sites that specify it - figure it wont do any difference on normal sites.