Need some advice.. help?

So I recently started school again and one of my best friends apparently started talking to someone over the summer. Obviously I support her even though I'm not a major fan of the guy, but what I just found out was that this guy also happens to be a guy my other close friend was talking to and had a "thing" with. My best friend decided to keep it from her until it got serious because she didn't want her to get hurt if nothing actual ended up happening, but it did get serious and my best friend decided to tell her and now my close friend is mad at her. My best friend keeps coming to me for advice but I don't really know what to say cause I care about them both... help please?

p.s. both my friends are best friends😩😞