<human>: Can you provide a brief rundown of the text in within 3 sentences?

Original post: 

Long story short: My GF[23f] told me [24m] a few weeks ago that our mutual friend, whom I'll call Cassy, [23f] cheated on her bf [23m] whom I'll call George. (My GF and I have been together almost three months.) Both of these two friends have been my friends since highschool. My GF has only known them both for a year, but my GF is best friends with Cassy, and Cassy told her about this issue in secret. Apparently, George knows about it, and is a dumbass and wants to "work through it."

My GF told me not to tell anyone, because she doesn't want Cassy to know she broke her confidence. But I'm not going to be friends with Cassy. Besides the fact that she cheated on George, she's also done a whole host of other things that makes me not want to be friends with her. So I blocked her on FaceBook, Twitter, instagram, the works.

Now her BF, George, texted me, asking what my issue is with his girlfriend. I'm not sure what to do.

1. I don't text back. I just stay out of it, never text him back. The problem is, is eventually, I WILL see them both in person.
2. I text him back part of the details. I tell him everything that bothers me about her, BESIDES the cheating stuff. But then this might make it awkward in public when we all hang out.
3. I just say fuck it and tell him, "BECAUSE SHE CHEATED ON YOU.", and hope my GF doesn't get too mad. Total honesty, all out there, and no awkwardness in public.

Which should I do?
<bot>: GF told me a secret that I acted on by blocking a girl on FB, but I'm being asked why, and I'm not supposed to know the real reason.