what the?

I’ve been hanging out with a group of people about once a week since I moved to a new town a couple of years ago.
A couple of months ago, a female in the group actively cheated on her partner who she had previously bought a house with, in front of me. I was shocked at her lack of remorse, and this unspoken rule of hers, that I wouldn’t say anything to her partner (also a friend of mine). So, I had to say how it made me feel. I explained I felt really uncomfortable going to their house, because I wasn’t sure how to look him in the eye, that I felt she was asking me to be an accomplice to something I really strongly disagree with.

I am still really angry, because she really doesn’t care how hurtful her actions are, she would be gutted if her partner cheated on her. She’s so darn selfish and self absorbed.

Why on earth is she still inviting me for dinner? She asked me to ‘forget all about it’…in other words, I had to shut my gob.

I havent been to their house since, and it’s not fair because I’m the one who is now lonely, and SHE is the one who did the deed.
She got angry with me, because I was ‘disloyal’ to our friendship!

ANyway, please explain why I still care, why I am still angry!

Thanks!