Mike effectively saved the girl in the drugstore from being hassled, and potentially from much worse. Note how he reports to Mark and Bryon what happened and how he ended up in hospital. He watched the gang as they increasingly became more and more intimidating to the girl, then finally when they physically touch her, he intervenes:
When one of the guys grabbed hold of her and really got crude, I got fed up. I went over and said "Let her go," like I meant it. They all looked at me for a while, like they were trying to make up their minds whether or not to jump me... They finally decided not to.
Mike therefore prevents the gang from doing anything further to the girl, and although it is not clear how far the gang would have gone, it is perhaps suggested that they might have raped the girl if he had not stepped in. He then takes care of her and drives her back home, when she turns on him and he is beaten within an inch of his life by her friends. This is how he ended up in hospital, as he tells Bryon and Mark, and all because he did good.