loza. Your Dad is an adult and has to make his own descision, albeit a bad one! In his heart he knows that his girlfriend was completely to blame, not the other guy 'taking advantage.'
I don't think issuing an altimatum is the answer, although I think you must have a shaky relationship at best with your Dad if you think that he would choose this flighty, immature woman over his own flesh and blood. That is far more worrying than the fact that he may take her back.
You don't say how old you are loza and I would make several suggestions if I knew this. As it stands, you really cannot do much except stand back and watch him make a complete fool of himself and just be ready to help him pick up the pieces. Hopefully though, he may see sense this time.