<human>: Give me the gist of the text in a nutshell

So I'm doing this from my phone at a bar. 

My father left today on a fishing trip with some friends. I confirmed that he's been sleeping with one of their wives every Thursday morning at my grandparents empty house (they own a winter house elsewhere)

Overall I think he's an entitled prick who doesn't appreciate his wife. He doesn't do anything around the house and thinks it's okay because "he does outside work my mom does inside work"

Total bullshit. She makes more money than him and works more hours. He's terrible with money (they both are, but I say he's worse) 

I don't know who to bring this up with. 

I could bring it up with my mom, "dad's sleeping with X, I think you should divorce him"

I could bring it up with my dad, though I don't want to

I could bring it up with the woman's husband. They have 3 kids, eldest being almost 18

I'm the youngest of my siblings and will be moving in with my girlfriend at the end of the month. 

Overall I know my mother is unhappy and lonely. I'm not sure what to do.
<bot>: father is a shitty husband and I think my mom should divorce him, I know she's in a rough place though with little family left