Husband's Girlfriend

Yes you heard it right. My dad is cheating on my mom. Idk how to react. I was shocked, hurt, disappointed etc. ALL the emotions you can possibly feel. They are still married. I am 28 yrs old. I was wondering if i should confront my dad by letting him know that i know or just deal with my own emotions. It is eating me up. I dont talk to him much since i found out. I told my mom she basically said, "i am tired of this. I dont care. I have you and my granddaughter. I am good. He can do whatever he wants." Tired because it's not his first. When i was little he did the same thing. But i was silent. This time i want to basically let him know that i know.