I tend to stay away from questions from folks who have never heard of Reading Rainbow but I figure you need some real help.
No, it's not okay.
What would his girlfriend say? Is it possible that she might want to go to prom with him? What would you say if you were his girlfriend in this position? Ultimately, its probably best if you just find your own dude who doesn't have one of those girlfriend things.
There's a simple rule when it comes to questions like this: if you have to ask, it's probably not okay.
Take that to the bank.
And go look up Levar Burton
Don't do it. IT IS NOT OKAY!!!
I was the girlfriend of this situation about 3 years ago. It did not end well, caused a lot of unnecessary controversy. If anything, take your cousin.
Doesn't the asker have any single guy friends to ask?
Okay, the fact that you have to ask sort of says that no, it's not okay, but guys... Sometimes it is okay. I went to a school ball with a friend who was being picked on by other guys for not being a "womanizer". No feelings attached except the sympathy for my pairless friend. My boyfriend didn't mind one bit, not even that we shared a hotel room. Yes, I asked several times and paid attention to how he'd react to "Yeah... we only get one hotel room, but separate beds, are you okay with this? Really, really okay?". It's all about if you can trust the person, and if the third wheel (in this case the question asker) has feelings for the one in the middle.
You should ask his girlfriend, not just him. Get to know her, give her a reason to trust you. I don't know why he wouldn't take her girlfriend to the prom, but I guess there is a reason.
See, if they went to different schools or he graduated the year before, there are legitimate reasons why he might go to this girls prom and not have plans to take his girl friend. If this was a situation where she knew the girl friend well, and there was a friendship and trust already established , this could be ok, but the fact that she had to ask, leads me to believe that this isn't that sort of situation, and it's a bad idea of she doesn't already have a good relationship with the girl friend.
My boyfriend did this to me. Took another girl to prom because she pressured him into it. I'm still mad at him about it, and will forever view her as a snake and a tramp. Even to this day, I still wonder whether I should just leave him. It's not easy regaining trust.
I hope you didn't do it. Trust me, you don't want some woman and all of her friends thinking you're a psycho, disrespectful bitch.