Well first let me say I’m sorry to hear of this unfortunate news. It’s hard to deal with this type of betrayal especially since you love both parents.
My thought is that you have a talk with your dad. I wouldn’t say anything to your mom. I would talk with your dad and let him know that you know and ask him to do the right thing and tell your mom. Keep in mind that he is grown and he along with your mom will have to work this through. Don’t go into the conversation with anticipation outside of telling him you know and how you feel. The outcome of the situation is between your mother and your father. If you think your mother should leave and you hold your position you could wind up being disappointed.
Be there to support your mom when she is ready to reveal. She may decide to just deal with it with just your father. If your mother comes to you again I can’t stress it enough to support her but try not to intervene. Matters of the heart don’t always work out the way we want. You will be angry with your dad know that as well. The image you have of him will forever be changed but you’re an adult you have to learn how to deal.
No doubt this will shake your family. Be prayerful, centered and allow your parents to work this out. Work on your issues separately as possible so that you can support each parent should you choose to. Again I’m sorry for this and will be thinking of your family.