I actually tend to agree with you as I've read through this thread because what struck me is that the DIL clearly needs help. Addiction is a disease, and she clearly has some mental health disorder going on. If I were the OP, I would take a position of encouraging the whole family into therapy so they can be healthier together instead of trying to drive seeds of wedges between them. The son will figure out on his own in his own time if he wants to stay or go.

I guess I just don't see why the DIL is undeserving of compassion and help. (I know that hasn't been spelled out but that's the vibe I'm catching.) You can't pick who your son loves. If you want to help and be supportive, encourage therapy for him and your DIL to work through their issues.

I'm not painting either one as a saint but judging her is the road to nowhere. Like or dislike, she's who your son has chosen.

How many people here displayed very erratic behavior before getting a diagnosis and help? Would we recommend that the DIL be treated the same way or use our experience to encourage treatment? I'm not absolving her of any wrong doing. She is responsible for her own actions. But, if she has a disorder and clearly has addiction, then she is not in control. And that means she needs help getting treatment.