Two reasons why you should do it:
1. Your friend only wants to go out with her to spite another girl, which is wrong if you ask me. Most likely their relationship wont last too long seeing as he probably only thinks her to be attractive and is doing it because he can.
2. YOU like this girl for what she is and what she is in to. You could be a good bf to her and if things dont work out, there is always friendship there.
Your friend doesnt seem like a good friend if he doesnt want you to go out with her so he can get revenge. Who cares what he wants man, what do you want?