Boundaries in Marriage

What’s the proper protocol for spouses when it comes to helping other people of the opposite sex with problems – especially problems of the marital variety? Sadly, my brother and his wife are in the process of divorcing. My sister-in-law called my husband the other night to see if he would meet with her to provide counseling and advice. I think this crosses a line – she should be discussing her problems with another woman, not my husband! He, on the other hand, feels that he might be able to help in some way. What do you think?