Its simple, but its not.
This girl has only had her relationships last for two months and a few days, all of her relationships. From what she has told me, her first relationship she ended, and she says she regrets it. Which gives the reason for the repeated time line:

Shes trying to fix the mistake, but is repeating it.
Or at least thats what I think.

So, the person she ended her first relationship with is back around and she says that she wants to put it back together. What shocks me is he bluntly told her wanted to get back together to have sex with her.

Currently she is with another guy who has been with her for almost two months. And she is considering leaving him for her first guy. (strange isn't it)

Where I come in is, telling her that there is no reason to leave a perfectly good relationship just because an older boyfriend wants to get back together with her, which probably won't work out anyway, and just leave her right where she left off so long ago.

But I don't think I was very convincing. So I need to think of something that makes enough sense so she doesn't make the same mistake again.

She's a dumbass.

This isn't your problem to solve. If she wants to leave a perfectly healthy relationship for this other guy whole told her, straight forward, he would only get back with her for sex, she isn't worth your time.

Maybe she thinks they'll resolve their problems and everything will be good again if he just gives he another chance, despite what he says. The only way you could stop her is if you made her some how realize that life doesn't work out like romantic comedies she sees on the television. Get her head out of la la land and down to reality so she can stop being such a silly twat.