I am positive I have caught someone cheating I just don't know how to go about telling the other person?

VERY LONG story.. I have caught my father cheating. I have confronted him about the issue at hand and he lied straight to my face. I have evidence I have photos and I even know others who have witnessed it happening. Only one issue the person he is cheating on is my mother someone he was has been married too for 30 years this year. The woman he is cheating with has talked to me about it she told me she knows I know about it. I have also been informed that she has spoken to my mother already and my mom knows. I have found out from more than one person my mom knows and chooses not to do anything about it. I know my father is an A$$ for doing this but I still feel I can't tell my mother for fear of betraying my father. But a part of me feels I should tell her in case she doesn't know. How do I do this? If she does know and I talk to her I could embarrass her. If I do tell her and she doesn't know I could ruin my parents relationship. I don't know what to do. I am lost and confused this is effecting my every day living having to keep this secret. I also have weighing on me that I know and if I don't tell her and she finds out from some one else I knew I could loose her trust. Can anyone help but giving me some words of advice or help me with what I should do.