How should I approach this?

This is a really hard situation. Well you need to talk to your boyfriend about this.(first tell him you trust him) Ask him whats up. Then talk to your friend and ask her too. Sit down with both of them if you can. Now there might be a chance he might break up with you. For this all you need to do is move on. What could also help with that is liking someone else. You're very young and most likely pretty. If your friend betrayes you shes going to get her spoonful of **** someday.