How can I let this go?

maybe it is just the way she talks to some guys, and just see's them as a close friend so talks differently towards them. you could try to ask her to step it down a bit when talking to him so that you become less paranoid. we all get the feeling at some point within our lives and its not good but if you keep getting paranoid she may start to get annoyed and then something worse could happen. just try to let it go and stay trustful if she does do anything with the guy she is not worth the pain.