Here's the thing.
You need to stop fukking with her. She's obviously a ratchet chick, and you're probably not the only side-piece nicca she has on the roster. Whether or not you stop fukking her won't really matter to her relationship; she'll just find some other dikk.
Would I tell the dude? That's a tough call. On one hand, I don't blame her for accepting the proposal because, well, lets be honest, no woman is gonna turn down a diamond and a wedding day. But the fact that she's still willing to fukk *after* the fact indicates a high-level of ho-ism. You'd be doing him a favor, although he probably wouldn't see it that way right away.
I say, if you do decide to tell him, best way to do it would be to blur out your name and face, and send him an image of the conversation on Facebook.
Good luck. Get tested too, btw.