In that case I think everyone should tune to the melodeon. If its in concert that makes life easier for everybody but even if its not, if everyone tunes to a different 'correct' note either you don't get to play your box or it sounds wrong when you do.

If your fiend refuses to tune to the rest of the session, he shouldn't try to join in. If its so out of tune with the rest of the session that its unbearable, or even messing things up for some people then ask him to stop (or offer him a tuner which seems to have the same effect. And don't feel bad about it - its him that's out of order.

You may still have trouble with the whistles. A lot of non-tunable whistles seem to me to be out of tune with each other and everyone else.