This is his mess, not yours. He needs to be a man, get a job, and support his child. There're really no choices for him, unless she chooses to have an abortion. The "getting back together" thing is between him and her, not you.
And, I would bet money that you don't know everything about their relationship. In my experience, most "crazy women" are the result of a propaganda campaign from the man, and they aren't nearly as crazy as the men who've dated them would lead you to believe.