Really getting under my skin

So I have a best friend, she got married 6-7 years ago. Now she no longer wants to be with her husband an is talking to 3 different guys an having sex with two of them, now her an her husband doesn’t have sex at all. But the new guy she is obsessed with an keeps calling me texting me all times of the night about it. Even bought him a expensive gift after the first date . But the thing is how can I break it to her that I don’t want no parts with it. Without hurting our friendship.i think it’s going to far. We are from VA