This beef is best handled between your partner and his mom. He needs to raise the issue again, and if she tries to get out of it by saying the conversation hurts her, he needs to stand firm and say that this dynamic is hurting him every day.
He needs to come right out and say, 'It's clear to me that you don't care about K, and I want to know why.' Sometimes people don't have the vocabulary to express themselves, so stating this fear right away might get the conversation going.
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