Hey man, I wasn't trying to make fun of the situation itself, just the way you put it. The situation sucks, obviously. And if you want my opinion, I'll give it to you.
My mom and dad haven't gotten along for the last 10 years. It has seriously warped my perception of "relationships" and marriage especially, just from being around it so long and seeing them hate each other, only staying together for the sake of me and my two sisters. I'm 99% sure that they will be divorced or at least separated within weeks of us all moving out. It might be something to consider if you've got younger siblings and you don't want them growing up with that.
If a man is willing to cheat on his wife, I personally wouldn't trust him with anything. So if you talk to your dad and he says he'll do something about it, that's bullshit. Talking to him would at least make you feel better knowing that it's out there, but most likely, nothing will change. The main thing I've always thought of is that IF my wife was cheating on me and my friend/son/whomever did NOT tell me, that would be a huge betrayal on their part. I would feel I deserve to know that my wife is breaking her promise to be faithful to me, so I would tell your mom if you can live with that. However, the last part is entirely up to you; nobody else can make that choice for you.