It's okay, these things usually work themselves out. I'd try not to feel guilty, as it is your friend did say he didn't like her, and you're not psychic so he couldn't really expect you to know what he was really thinking. But, it's really the girl's choice, if she wants to go with you to prom, then so be it. Your friend will get over it eventually, just try to be nice and smooth things over with him.

And, you'll be dealing with this stuff forever (seriously, I have Alzheimer patient dating stories I could tell you).