I divorced my first wife and got full custody of our kids because she got heavy into drugs after our 3rd child was born. She went to court-ordered in-patient rehab so she could get partial custody (every other weekend and summers). I paid for the rehab because she couldn't. Cost me a little over $2,000, and she seemed to be doing really well when she came to see the kids several months later. She looked and acted like the girl I married.
So, yep, I asked her if she wanted to spend the night. It was like old times except for the fresh needle marks. Plus, I know damn well she did a line in my bathroom, so I sent her home.
I called my attorney next day and he got a court-ordered drug test pretty quick. It helped that I lived in a small town, but the judge also saw that she wasn't showing up for the rehab's "maintenance" drug testing.
Bunch of stuff went down after that, but to be brief, that was the end of my relationship with my ex-wife, obviously. And after that, whenever a relationship ended, it was final no matter which of us broke it off. Even whenever she's said "We can still be friends, can't we?" I simply do not look back.