I still use my dad's HBO GO account (I know, grow up, right? :oops

but each person in our family has an email address from the cable provider. I thought I was logged into my email, so when I saw I had a new message, of course I clicked and read it. Except it wasn't my email. It was my dad's because I was using the HBO account recently, and it was from a woman who is very much NOT my mom. Before I figured out it wasn't for me I read something like "I tried to get out *to the store* earlier but she wanted to come! Damn! I will talk to you tonight. Miss you and love you so much" I closed out before I could read anything more. Putting two and two together: my dad is having an affair. Slowly the pieces came together; she's the paralegal/secretary at my dad's firm. They go on "business trips" (now thinking back to when he was telling me about the "conferences" he was going on makes me want to throw up) and I have heard her name a lot. I just can't believe it. They've been married for 35+ years.
Also: before the anti-snooping people come out, 1. I really don't feel like I snooped. It was completely honest and even though there were DOZENS of emails I barely read two sentences before I closed out. I want to throw my laptop out the window, ugh. 2., He freely and willingly shares his login and password with me (and my sister!) to watch HBO GO so I am shocked this is happening in a place we can so easily access.
Of course he's an adult, we're all adults, but I am so worried about my mom in this. If they get divorced, she will be so screwed. He's in biglaw, she's a special ed teacher. They have a mortgage, my dad's student loans, CC debt... there is no way my mom would be able to hire a lawyer, let alone survive financially if they got a divorce. Do I talk to him about it? Warn my mom? Send an anonymous email? This just happened like 10 minutes ago so I am completely freaking out. Help please.