My girlfriend was cheating on her partner with me. Should I tell him?

Note: everyone in this scenario is in their thirties

I've been seeing a girl for a couple of months, we've slept together a few times and things were getting serious. Yesterday, she confessed to me that she's actually been in a relationship with another man for the last 4 years and that she wanted to leave him for me.

I told her to piss off because if she's cheated once, she's just as likely to cheat on me next.

I don't know the man she's with but I was able to track him down on Facebook easily enough and now I'm wondering if I should tell him what she's doing.

On the one hand, it's not really my business to interfere in their relationship.

On the other hand, if it were me, I'd want to know if my partner was cheating on me so I could stop wasting my time on her.

I'm not looking to get revenge on her or anything, I just feel like telling her partner is the right thing to do but I'm not 100% sure on that. I'm also worried about any blowback that could come my way because even if I tell him anonymously, she will know the info came from me. She knows where I live and work, who knows what she might do if she gets angry enough at me. It's not unheard of for exposed cheaters to falsely claim they were raped, for instance.

Edit: I told the guy, he cried a lot and broke up with her. She keeps calling my work but the receptionist just mutes her calls and leaves her ringing.