John - how did you keep a straight face when he turned up with two?

Seriously, I think that its always a good thing to encourage and respect people's contributions irrespective of how unformed their talent (hey! how tactful is that!). But its not acceptable for someone to trample over a whole night (even if they're good & perfectly formed talent-wise....).
So whatever his quality and choice of instrument, he should not be joining in uninvited & persistently.
Something needs to be done, for the good of the club, the singers and the individual himself.

Keep us posted....

Kris