Maybe you could enlist your mother's help - ask her to approach the eejit and tell him she needs the bodhran back (insert any old lame excuse as required - bird-bath, panning for gold, serving cous-cous...)
Failing that, how about strength in numbers. Have a quiet word with the other singers, and if the pillock starts beating off, get everyone to come out with a collective groan and a few glares in his direction.
Trouble is, it's amazing how thick-skinned some people can be (...the bodhran's is probably thinner...!).