Thanks guys. I've come up with an idea that I'm hoping he will be somewhat responsive to:
We will swap weekends so that he will have her the weekend beforehand. I will bring her to the party (even though it's an hour and a half drive each way), and I will explain to him that the idea is so that he can enjoy his birthday with his friends without worrying about additional responsibilities. We will stay for an hour or so and then leave. I think that's the only way to avoid trouble.
The party starts at 1. I just have to pray that he stays sober enough until we leave. I hate this sub-par life we live. Forever stepping on eggshells to avoid the alcoholic beast. Makes me sick to my stomach.