Amazing advice above from both ladies! I second SM's advice completely! Sorry you're dealing with this, I'm sure it's so hard. I hope your Mom doesn't put up with this for too long without some kind of consequence (even a separation) and I hope your Dad comes to his senses. Sad situation.
I agree that while you will be supportive, try not to get sucked in too much where this becomes YOUR problems, because essentially, it isn't. I would encourage your Mom to also lean on others, her peers, people who wouldn't be in the "middle" as you likely feel you are.
Take care and good luck!
Ugh, so very sorry to hear this. Thing parents do can be SO disappointing!! I'm sorry you and they are going through this! My best advice to you is to be a listening ear when you can but to also have your own boundary. Don't take this on as your problem. It will be very easy to completely pick your mother's side as she sounds the innocent but also--- she chooses to stay with him even though he is basically having an emotional affair and wants to amp it up to a physical one by meeting up with the old girlfriend. Yes, he's being honest as many would just do that behind their back but she now has some significant information on him. If she chooses to stand by her man, it's then not fair to constantly complain to her daughter.
but realizing this is new---- just support her through listening. I'd throw out a couple---- tell him NO comments. That he leaves you if he goes to see her and there will be no one to come back to. That she should expect him to cut off all ties and if he says he can't then the relationship should be over. But you can only tell her that a couple of times and then it is up to her.
My parents divorced when I was in college. My dad was a shmuck. But, he's my dad. You can not be placed in the middle of their issues or try to solve them for them. That is very unfair to you. Your step dad is an adult making choices even if they are bad one. You can tell him clearly that you believe he is making a mistake leaving the family like this but I would really do your very best to let your parents sort it out rather than your getting too involved.
I'm very sorry that your mom is hurting. I've been there and it is terrible to see the rock of our own life fall apart when their life hits a patch like this. If she seems to be getting depressed, do encourage her to see a therapist. That is a really good thing for her to do--- a neutral party to vent to. She then, if it turns out she drifts into clinical depression, should see her doctor as depression is treatable. We went this route with my mom.
Best of luck to you. I know how hard it is. It is also good to get out of the house and to remember---- this is THEIR relationship. You must go about your life without this feeling like you are a threesome and he's left you too for another woman. It feels that way but he hasn't. good luck
Since you do look at your step father as a father I take it you may be close to him? If so, can you ask him to sit down and talk with you about this? I think you're right and this thing could possibly being infatuation. He's a grown man and will do wants he wants to do but maybe if you talk to him about all of this and how it will effect your family he may think twice about it, hopefully.