Absurd in a common sense absurd way, but if they were not bringing the man up on charges related to her death, than the jury can't consider it. The judge would have instructed them on that point at length. Of course, the jury members aren't idiots, either. They know the girl is dead (otherwise, she would likely have been at the trial). So they no doubt did take that into account -- quietly -- as they deliberated.
Oh, I know it is correct in a legal sense, but frustrating in the "it's obvious that the rape caused her to kill herself" sense. Actually, I'd bet that the rape was just the last assault in a years-long series of assaults against her by the father. Most kids don't up and kill themselves after a singular event. Poor thing.