I think my first question would be "why not go to the cops." If she lacks the evidence to do so, the question becomes "did he do it" or not.
I remember reading a story out of Connecticut where a wife told her husband her daughter was molested by the neighbor. Enraged, the husband went over to the neighbor and stabbed him to death. When the story was dug into further, the police believed that either the daughter (who was 2) was not telling the truth, the wife was lying, or the wife badly misinterpreted the daughter's statements (again, from a 2 year old). There were a bunch of problems with the story, such as the neighbor was disabled yet he somehow was able to get a ladder into the 2nd story room the daughter stayed in. So now there's one dead guy, one grieving mother of the guy (who found her son murdered), a father who is in prison for murder for the most formative years (he got 12 years...an extremely, extremely light sentence IMO), a daughter who will spend most of her childhood visiting her father in prison, and a mother who IMO caused the whole mess yet somehow got off in all this.
Sorry, didn't mean to turn this thread into that much of a downer, but that's the first thing I think of when I think of vigilante justice.