Yeppers, they find ways to hurt themselves when there is nothing in their paddock but the air they are breathing...silly beasts. EDIT: Forgot to ask, Is she in with other horses? That really looks like another horse took a bite out of her lip...
It is amazing what these guys can do to themselves and never even seem to feel it, and they heal remarkably well and fast. It looks like a pretty clean injury and if you follow your vets advice she should heal up just fine in no time. She will certainly carry a scar, but I wouldn't foresee any major complications. EDIT 2: Forgot to answer you question about ointments. I would just follow your vets advice, around the mouth there isn't too much you can apply that they won't ingest. If the flies bother it, you may be able to get a stick of Swat! Fly repellent and put it around the area.
If her current hay is on the coarse side, you may think about getting some softer textured hay for her to eat for a bit to keep from aggravating it as it heals.
Good luck with her, maybe you could keep us updated with pictures?? It's always educational to see how these things play out.