I don’t have any advice, but I’m very sorry you’re going through this, and sorry for what happened to your dad. Being betrayed like that really does a number on most people and it’s incredibly tough to move on, even for most people who eventually made it to a happier place, psychologically speaking.
The best thing for your dad to do, though I doubt he would, is to break off contact with your mum and block her on all his social media etc. That would be a good first step towards him healing. If he has any hobbies or interests would he join a group of some kind so he could meet other people?