What to do

This is about My very best friend from middle school.... we have been like sisters forever and she was even my maid of honor at my wedding. Well I know for a fact that her boyfriend is planning to propose. She has always been a little... well slutty... anyways so she talks to other guys 24/7 deletes the convos from her phone. Even hangs out with some of these “old friends” once in awhile. But swears it’s nothing sexual but I find it hard to believe because if your having to hide and delete it then there’s obviously something you don’t want your man to see. She claims it’s because he would be mad if he knew she had male friends. But seriously wtf? I was with her the other night and this guy was calling her and texting at like 2am??! While her man was at work. She didn’t answer the call and I assume it’s because I was around but I saw she was messaging the person back.... I just really feel bad because her boyfriend is a really good guy and since he is wanting to propose I feel like he should know something! But I don’t want to loose my best friend.... what do I do???

Also I might add they have been together 4+ Years