Marle, I'm sorry you and your family are going through this. I know she's on subs so she isn't going through any withdrawals, but she may very well be depressed. It's the easiest explanation why she wants to be with the bf. That and hormones. They aren't supposed to get involved in any new relationships but old and familiar is any easy out.
I read she's taking Chantix but is she still taking Seroquel? Maybe a trip to her doctor and an adjustment with her meds is in order. Hopefully she'll do that and ASAP.
My thoughts and prayers are with you.