Friends boyfriend said he would "get rid of her and whoever she cheats with" what would you do or say in this situation?

They been together 3 years, apparently he's been cheated on before and said he couldn't handle it and would probably go overboard if it happened with her, she's his longest as well
Friends boyfriend said he would "get rid of her and whoever she cheats with" what would you do or say in this situation?
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