Best friend is doing something with guys I do not like?

I have the problem described above. My best friend, we've known each other for about 2 months, most likely doing something on the weekend with a boy who does not like me. I'm probably scared that I'm not so important to her anymore and that something happens when she's with him. (Cuddling, kissing etc…) I do not know what to do, do you have any ideas?

Talk to her. Ask her what she wants from the guy. Tell her why he does not like you & how you feel about it. With something like that helps only talk.