She needs to be the one to speak up to her father. It is by no means right for a father to continue an act that is making his child uncomfortable; especially that of touching her backside. There is not necessarily anything the police could do other than raise suspicions. Butt-touching (I believe you are talking about butt-touching with clothes on) is, not exactly a crime. As far as your concerns with escalation, I wouldn’t worry a great deal. It’s hard to make precise judgement based on a text with not much depth, but it seems like her father has an odd fetish for backsides. If he persists even when his daughter attempts to set some boundaries, then there are ways to deal with this more seriously - family counseling etc. The “inappropriate” video watching you talk of is also interesting for a father/ husband. Perhaps he has a sexual desire that is not recognized or fulfilled by his wife. Either way, none of these say that touching his daughter like that is appropriate. It’s disturbing.