Is there any way I can tell my best friend's boyfriend that she's cheating?

Here's the deal: My best friend has a long distance relationship with a guy I've never met or spoken to. Now she's boasting about having sex with this other guy she just met a few weeks ago, and has told this guy she has feelings for him.

Whenever she brings up this new guy, I basically yell at her. I tell her I do not tolerate cheating because it's something only scum could commit. I've seen so many people get hurt by it, and the fact that she's cheating is deplorable. I tell her she must either stop doing it, or break it off with her long distance boyfriend.

I can't tell you how many times I've gone over this with her. It's to the point where if she brings it up, I'll shoot her down immediately and say, "you know how I feel about this.". Because I disagree so vehemently.

Talking to her will not stop her. She obviously wants to have her cake and eat it, too. All the while, this poor guy hundreds of miles away is telling her he loves and adores her, and she's saying it all back and stringing him along.

Do I have the right to tell this poor guy she's cheating on him? I personally don't know. It's a major blow to my best friend, but I feel like this guy will never find out and I'm almost responsible for keeping it from him.

How she doesn't have a conscience is beyond me. I'm starting to pull away from her as a friend because I don't agree with her disgusting actions.

What do you think I can do? Should I stay out of it and leave the guy out of the loop? Talking to my best friend about this is out of the picture- I can't even tell you how many hours I've spent lecturing her about this. She won't stop or change.

Thank you for your feedback!
Updates:
+1 y
(1 of 2) I think the real motive behind me posting this question was to write it all out and get my feelings down. I'm very passionate about defending the weak (in this case- the boyfriend being cheated on) but in this particular situation I don't think I have a right to poke my nose where it doesn't belong. Perhaps if he was also a very good friend of mine I would have more of an obligation, but I've never met the guy before. I've decided I'm going to talk to my best friend and tell her if she
+1 y
(2 of 2) doesn't choose one guy, I won't feel comfortable associating with her because her actions are not acceptable. Thank you guys SO MUCH for your answers, and (for the most part) being very sweet and understanding.
Is there any way I can tell my best friend's boyfriend that she's cheating?
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