My friend and his wife had to do that with his wife's mother. She said the same thing as this lady about one of her other kids marriage "I will make sure he divorces her." She tried to break my friend's marriage up as well, which was his wife's come to Jesus moment: "Wow. My mom almost manipulated me into divorcing the man I love. None of this is normal." They ended up getting couple's counseling, and she went almost zero contact for a few years.

In my own family, my SIL is a narcissist, and we can already see her training her golden child to be just like her. Meanwhile, the other kids seem to be adopting some strange coping skills. It sucks. People like that do so much damage.