That is exacly what I am going through and the boy messes around with me and my friend behind our backs but we tell eachother everything. All we did was talk about it and we came to an understanding of who is going to date him and we were all fine. So, I think ya'll should just talk about it.
This happened to me back in highschool, but my friend had already dated the guy, then they broke up and she ended liking him again at the same time i liked him. But my decision was to just move on from that guy and let her have the crush, eventually we both moved on and nobody ended with the guy.