It would really depend on the kind of guy I was . . . If it were me, as in, The Mystery Man, I would be hurt and confused but wonder what kind of mess I had gotten myself into with an obviously unstable woman.
If I was some dude who cheated on the baby mama or was prone to prison sentences, excessive drug or alcohol use, abusiveness, or just a general total screw up in life, I may think that perhaps it's a good idea that I'm away from this poor kid who didn't ask to be born and especially to have such a lousy father. One can be a loser and be self aware.
You don't say what the scenario is, so it's hard to be more precise than this. Good luck!
Not knowing the full scenario and knowing some of the psycho chicks I've met over the years, I'd ask the girl if she was just doing and saying all of that to see if the really great guy would listen to her and stay away or come chasing after her like a puppy. If that's the case, then she needs to get her head out of her butt and stop testing the guy. If he has any self-respect, he won't chase after her and she'll lose him because she was an idiot.
If the guy is the loser, jerk, why are you asking the Mystery Man what he would do? It doesn't matter. Once you leave town, change your name and hope he doesn't find you.
Since the question asker is asking what another guy would do, I'm leaning toward scenario #1.
Who cares there's a child involved-- both people need to put their own interests aside and do what's best for the child if she decided to keep it.
An upright man would get a DNA test when the child is born. If it is his child he will provide regular monthly child support for his child and get a court order outlining custody and visitation issues.