Max,
Sounds like you've already had a heart to heart conversation with her, but she needs another one. She has said it's none of your business, but she involves you by telling you about it. If she really wants what is best for her children, she will leave and get out of this relationship.
Do you know her family at all? Would you be willing to break her trust in you by discussing the issue with her parents, brothers, sisters, etc? Maybe the more people putting pressure on her to do something would provide more support to actually get rid of this guy.
Best of luck, you sound like a good friend to her. |