what do I do?

Well...this is a hard one. There are lots of things to consider. First, if you guys are TRUE best friends then tell her. Just tell her. But remember, she has feelings so don't it mean. Also think about if she loves him or likes him a lot or what. Also think about how he feels about her. Because if she loves him and he loves her, then you could tell her but also be her friend and realize he is her boyfriend and they love each other. But if they aren't in love or like each other A LOT then yeah. I dunno. I had a lot, but it's gone now. lol. Anyway, tell her but just keep in mind how she feels about him and how he feels about her. Because it might not end up that great if you dont' consider everything there is to consider. Trust me. I hope I helped and if I can help more and/or you just want to talk, please e-mail me at ismene86@comcast.net.
Luv,
Amy