So - long story short:

We were best friends for over a year. We spoke online every single day and saw each other most weekends.
She started "dating" a guy, they'd been seeing each other for 2 months but he refuses to make it official or "put a label on it". He started messaging me on a dating website and wanted to meet up and I found out he was doing it to several others.
I ended up having to tell her as she kept saying she just wanted to know if he was serious or not. She thanked me, and thanked me a few days later saying she would have also told me if the roles had been reversed.
Since then, 20 days ago, we haven't seen each other and she's not even really spoken to me.
So I've basically lost my best friend because she made me tell her about her cheating non-boyfriend. She knows it's true because she saw the messages.

Just wondering what you guys think I should do about it?
I feel like giving up and leaving them to it. But I kinda just want my friend back, I don't care if they're together or not although she deserves better.
I at least want her to realise how much she's hurt me by all of this when I was only looking out for her and was kinda forced to tell her. Seems like a waste of a friendship!