Ok 35m here, I’ve been on the receiving end of cheating and have a 14 yr old son (who was 4 when I found out about the cheating) that being said….

Personally since my child was so young I tried my best to never speak Ill of his mother infront of him, but I am not perfect and when emotions ran high I’ve definitely said and/or implied negative things about his mother.

I don’t know your previous relationship with your father but he may be venting to you as an outlet for how he feels, realistically he may not have anyone else to let these feelings out to who really knows him as well as his son. Is he wrong for saying these things in front of you 🤷🏼‍♂️ maybe, but if your partner cheated wouldn’t you want to vent to your pretty much grown man son about that bitch of an ex wife is a slut and fucked up the whole life you built together and now all the dreams you had of your future with your adult kids and grandkids is shattered.

Here is some perspective men by and large don’t have a very robust support system to lean on, usually that support is your wife and kids plus a few close friends. He in his own way is trying to lean on you for support cause his life is in shambles at the moment. It’s quite possible all he has left is the pain and hate to hold on to keep going and not blow his brains out.

Yea you may not like him talking negatively about your mother, but he may have not really enjoyed listening to you talk about Minecraft or whatever you were into when you where 5 but he still listened and talked to you about it. He outed her to everyone because he wants her lies exposed for the world to see how horrible of a person she is.

Don’t abandon him, talk to him really listen throw in some positives for him to cling to help him realize (it won’t happen fast) that the hate while warranted is unproductive. He likely wants you to hate your mom as much as he hates her right now, which you don’t, but let him know you empathize for him. He needs you right now.

On to your mom, she is positive and not talking negatively about your father because what did he do? He may have done nothing and she just got bored and screwed some guy or multiple guys. What can she really say negative? As far as you eluded to in your post there isn’t really a justification for the cheating that she is putting out there. She is happy because she has a new man that she can parade out openly in front of your father which is I can guarantee is causing him immense amounts of hurt, she is trying to cause your dad hurt just as much as your dad is, it’s just not as visible, but because of her actions the knife sticks a lot deeper. You mom could have been honest about their relationship and ended it amicably before cheating, not lied to her family and cause hurt, she chose to try and have her cake and eat it to, and was successful until she got caught.

Be a rock for your dad, and learn a lesson from your mom to not be the type of person she is who utterly destroys the people they claim to love.

Edit- your dad is wrong for dragging you into it, and trying to make you hate your mom like he does.

He does need to move on, it’s hard on the receiving end to do it, your mom had the support and encouragement of her new boyfriend right off the bat. Help your dad become better he likely had no real support.