J.C.
I can see both sides. When I throw a party for my child I put a lot of effort and thought into it. If someone came and right from the start it was clear they were intent on ruining the good time, I would say something to them. She should have given him a chance to correct his attitude before going straight to "leave", but he was wrong in what he said, and she was trying to protect her child from that negativity on his special day. He has apologized, and she accepted, so I would leave it at that. If you insist on pushing the issue and forcing an apology from her she may not be willing to give, this whole situation could get even more out of hand, and this one party may not be all that you guys miss out on.
I have one question, did he try to apologize at the party and request to stay, or did he just leave. If he just left, than I do not think she forced him to miss it. If he tried to make amends and she still made him leave, than she may have something to apologize for.