No you're not wrong for wanting to keep your dad in your life. Any reputable family counselor would say your mom shouldn't be asking you to pick a side. Your mom does need support though, but it isn't your job. What your dad did sounds really hurtful to her, as it would be for most people. Maybe he is your best friend, but being a parent is more than being a friend. It means being a role model & wanting to teach things like honesty & respect, especially for the people you're supposed to love.

Has your dad offered to pay for counseling for you to process these things & talk to a professional about? It sounds like you could probably use it. I don't know how old you are, but given that you're asking the question here & the way you're asking leads me to believe you'd be much better off talking to a professional about it.

When you choose to forgive someone you're not necessarily forgiving their mistake. It's not condoning, it's giving someone another chance to do better. Forgiving and protecting against risks isn't mutually exclusive, it just takes more effort to do both.

Your parents have some beef they need to settle between themselves - not with you. Any parent requesting their kids to retaliate against the other parent is a red flag. It's manipulation. Beware of her !

Now, if your dad had done something to you and you felt like cutting ties, or perhaps your dad represents a serious threat to your mother and cutting ties is necessary for her safety, well that's a different case scenario. But that's not what you are telling us.

TLDR: you are not a bad person.

Okay so... your dad is unequivocally a piece of shit. You said he's like your best friend, but are you okay with your best friend being a piece of shit? There need to be social consequences for being an unapologetic piece of shit (which one would need to be to have an affair with their personal assistant and then move in with her). Just business as usual isn't gonna cut it (think if instead of cheating he'd come out as a Nazi) and you would be condoning his actions if there aren't negative consequences of some form for this fiasco, though how much you escalate is up to you.

Edit: I have to say, the attitudes some of y'all have about parent-child relationships range from ungrateful to absolutely deplorable. Like, seriously if I heard "it's the parent's job to emotionally support their children, not the other way around" from someone in real life I wouldn't let that person within five miles within anyone I care about.

No. That doesn't make you a bad person. Frankly, I think it's awful that she's asking you to, but I imagine she's going through some things herself at the moment.

I think you have the right take here already- your relationship with your dad wasn't the one impacted.

Why is your mom the one that moved out? Also, the affaire partner moved in?

He's your best friend, so you three like hang out and watch movies together as a new family while your mom sulks in an appartment alone?

Not only does your dad suck but so do you. Maybe one day, you will get cheated on and understand the betrayal it feels like. Hopefully, you wont have children to drive the knife in even deeper. Relationships are suppose to change with a parent who has an affaire, it's abnormal not to be angry at what he did to your mother.