And she knows you're dating him? If so, I would just talk to her about it. If she's not planning on doing anything, I would just kinda leave it after you talk to her. If she is trying to get him, you need to tell her that you and him are together, and you'd appreciate it of she kinda laid off.
it is really wrong to date your bestfriends boyfriend. so if she is a true bestfriend she wont go near your boyfriend. tell her she needs to find someone else. he is already taken by you. tell her there are plenty of guys out there dont ever mess with the ones that are already taken! it will ruin yalls friendship in a heartbeat