I would never encourage someone who has not had minimal training to throw a punch at someone's solar plexus. In order to wind somebody the punch has to hit the right spot and it has to be a solid, hard shot. Doing this will throw an inexperienced fighter off balance. It will also open her up. Odds are that the dirtbag has been in a few scrums and can block the punch, then take advantage.
It would be better if she kneed the guy in the groin, or turned around and kicked the guy in the kneecap. This isn't a fair fight. Eyeballs are always a good target.