Mother’s emotional blackmail is toxic

DEAR ABBY: My husband’s mother recently told him he no longer matters to her because he is an atheist. His mother is supposedly a Christian, but she rarely acts like one. It has left my husband devastated and feeling more lonely than ever.
I can’t find the right words to comfort him when he’s going through something I haven’t got a clue about. How do I empathize with him to let him know he did nothing wrong and that he never deserved to have those words thrown at him by his own mother?
It infuriates me, but I don’t think it would be right for me to step in and talk to her directly. How do you deal with a narcissist who constantly plays the victim even after all the support you’ve given to her, financially and emotionally?