What would you do?

A few months back, I met this guy through my best friend. We both like each other and want to start dating. However, I'm hesistant to go out with him b/c my best friend actually likes him a lot and has for a very long time. I kinda feel like going out with him is a betrayal to her. I'm also hesistant to ask her if it's ok becuase she'll tell me it is even if it's not. I really like this guy but I don't have much friends and I know from past experience good friends are hard to find and she's the best friend I could ever ask for. If you were in this situation, what would you do?