I'm so sorry for your hurt.
My exabf went off partying with his ex shortly after we split.
I confided in a friend yesterday as to our split & my friend said he treated me like dirt & I deserve better.
It's so frustrating I know when you try so hard to get them towards sobriety only to lose them & hear they're with someone else, sober or not.
I don't think this is a real relationship she has anyway, I think it's a distraction & a fantasy.
When I went through drug rehab 6 years ago the counsellor told me that it is common to either replace a drug with another or to jump into another relationship which is what my husband did.
Big hugs to you.
We are here for you.