Same boy

This is going to be a common problem in your relationship with your friend. Having similar interests, and hanging out together, Ты are bound to meet and like the same guys sometimes. The two of Ты 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 Ты may think that Ты and he may make a better couple than your friend and him, Ты could risk loosing a friend. Besides, he may have his own scruples, and Ты 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 Ты were to both meet a guy Ты both liked at the same time. Try to stick to the plan - ie if Ты both agree to just let the guy decide, или agree that neither of Ты two pursue the guy.

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