Same boy

This is going to be a common problem in your relationship with your friend. Having similar interests, and hanging out together, you are bound to meet and like the same guys sometimes. The two of you have to set ground rules:
1. Should not be invading each others territory. If one has already started dating / having a relationship with the guy - do not interfere. No matter how much you may think that you and he may make a better couple than your friend and him, you could risk loosing a friend. Besides, he may have his own scruples, and you could loose them both.
2. Discuss the situation with your friend. There is nothing worse than finding out later that your friend had feelings for your boyfriend. The feeling of betrayal could result in loosing a good friend.
3. Discuss with your friend a plan of action if in the event you were to both meet a guy you both liked at the same time. Try to stick to the plan - ie if you both agree to just let the guy decide, or agree that neither of you two pursue the guy.

Anyway, without knowing more about the actual situation, that is the best answer I can come up with now.