Actually she should demand the real father to be responsible what. Why on earth that guy just piak and dump without any responsibility at all.
While I don't agree with abortion, I don't think you should be the one tanking everything and cleaning up the mess either. Unless you're mad in love with your ex, taking pity should never be a reason for marriage.
Having a kid with 2 fathers is already a headache, what if she later on says she doesn't want to have any baby anymore, you'll lan lan raise another man's offspring while you yourself doesn't have any.