
I'm going to look at this from the other side, re. your last comment - about them being childish. If there are lots of people going, then as you say, they don't need to talk. If your friend (the girl) is a good friend, then she should be willing to go anyway as it's your special night. I've had to do it before - my friend's birthday a few years back, and my ex was there. And no-one else! Just the 3 of us. I knew it would be uncomfortable - and it was - but I went, because it was her day and I knew she'd be hurt if I didn't.
I can understand the avoiding each other thing - that was the last time I saw him and I've only just started speaking to him again, after about 3 years. But to put that in the way of your birthday is selfish IMO! I would explain to the guy what the girl has said, and I would say to the girl that you want her to be there because she is a good friend. Then I'd leave it up to her.
I'm dreadful for trying to keep family and friend ties going despite adversity - but I take a tough stance with stuff like this. Some things are more important than a bit of psychological discomfort for one night!