Caught fiancés dad cheating on his mom again what to do?

So me and my fiancé have been together for five years. We have a 2 year old together. I am very close with his family and treat them closer than my own family. Well my fiancés dad has already been caught cheating before but we thought he had moved past it and was getting better. He was sleeping with other women and bragging about it to people in town he's friends with(we live in a small town so word travels fast) but he doesn't care he has no issue speaking his mind about his feelings towards polygamy and how he agrees with it and how he thinks women are beneath him. I honestly hate him but put up with him because he's my fiancés father. But we all thought he had stopped cheating and gotten better especially since he finally started to be caring, attentive, and loving towards his wife(my fiancés mother). Well about a month ago they both got smart phones and since they aren't very tech smart I told them. I would help with any questions they had. Well one night his dad asked me to look at this backup app on his phone and tell him what all it could do and what it was for. So I took a look and little did I expect to have every picture in his phone be backed up to it and there was a fully body picture of his dad naked him taking it. I was sick to my stomach as I knew he wasn't sending them to my fiancés mother. I told my fiancé but he got upset at me for wanting him to tell his mom. He says he doesn't want to tell her because he doesn't want them to fight again like they did last time. But I think it's wrong to leave his mother blind and believing this man is changed. I don't feel like it's my place to say anything but I just know this is wrong. His mom was going to leave his dad when she found out he cheated before but my fiancé was under 18 and still living with them and she didn't want to for the sake of him because he told her he didn't want her to. I don't know what to do. I can't stand knowing he's doing this especially when his mom works her butt off while he sits at home wasting money that they don't have. I need advice please!!