Ooof, tough situation.
I'd probably attack it like this:
- Make sure she's on birth control. This is the absolute first step.
- Sit down with her, and ask her questions. Don't put your opinion in any of them, try really hard not to sound accusatory.
- Ask why she was sneaking around.
- Then ask why she thinks that you wouldn't like him.
- Then ask why she thinks you think you wouldn't like him.
- Ask if she thinks that he's a good person who will love her and support her.
- Ask why she's different from baby mammas one and two.
- Ask how she would feel, and what she would do if he ends up back in prison. When she says "but he's changed", ask why she thinks that, and ask what would happen if he was a victim again and ended up back in prison
- Watch closely for any signs of abuse. Watch extremely closely for anything that would possibly violate his parole, and get friendly with Mr. Parole officer. There likely has already been a parole violation.
- Optional, but I like to look at financials. Show her the life that she can expect with him. Assume that she doesn't graduate college, and assume that both of them are working minimum wage or similar jobs. Then show her how much will be left after his alimony and child support checks. And make sure she knows that those child support checks are the absolute first priority for HER and her bad-boy if they end up together. You've heard of child support and how lousy it can be, right Jeremy? Ask her if she likes the idea of living in a trailer park, and be honest with her--that this is what she is signing up for because they won't be able to afford anything else.
Edit: The reason that last step is optional is because that one is giving advice. The first 3 are not. #1, you shouldn't have issue with. #2, you're making her state HER opinions. You're not stating yours. Just keep asking why. Tell her you want to learn what she sees in him so you can see the same. Hopefully she talks herself into "holy E36 M3, this dude is a loser". #3 is completly passive and just observing. the 4th, that is where you'll get pushback and rebellion. So tread carefully.