Tell his son or daughter that he made a mistake, apologize for setting a bad example, and tell them the right thing to do, assuming the relationship was untenable, would’ve been to end things with his wife before pursuing someone else.
Edit: A bad husband doesn’t necessarily make a bad father, though.
Edit 2: Come to think of it, a good father would probably also have waited a little longer before having his girlfriend move in.
A good mother will prioritize the wellbeing of her children over winning a conflict with her (ex) partner. It is completely unfair of her to ask you to pick a side or go no contact with your father. Hopefully she’ll realize this once she’s had some time to process her (understandable) emotions. In the mean time, best of luck to you!
NGL, IMO your dad sounds like kind of a piece of shit (based on the limited information available to me). However, at the end of the day he is still your father and it’s not your job to punish him for that. That’s your mother’s lawyer’s job.
Be there for your mother if you feel up to it (and you love her), but keep in mind that it’s a parent’s job to emotionally support their children, not the other way around.
Things aren’t much better in The Netherlands. The PVV is part of the government now. The good news is that having to form a coalition with other parties, seems to have reigned in some of their worst tendencies. Also, their minister of immigration is a total nitwit that can’t get any sort of actual policy implemented. If they keep failing to make any kind of progress on this issue, hopefully they won’t do as well in the next election.
The Netherlands, Germany and Scandinavia have all those things and people tend to speak English really well.
So she never accepted you as you are and didn’t support you when you needed her most? Your dad’s no saint, but if I were in your position, I’d sooner end contact with my mother than my father.