Don't know how much help I've been, but I wish you guys good luck and Merry Christmas -- despite Mr. Scrooge!
Were the events successful? I think the problem lies in the fact that he promised her 50% of the gross instead of 50% of the net profits. Is it possible that these terms were confused by either party? If sales don't look good, he could be looking at a loss. I think a lawyer letter will do...it will cost him twice that to have representation in court. Ideally, you should try to come to terms outside of the court if possible. Maybe some kind of payment plan if the guy is strapped for cash which it sounds like he is.