Should tell her she could do better. Like the response to one-sided non-monogamy like this should be to start finding his replacement and laugh when he comes crawling back after doing God knows what with other people. When it happened to me, I just ended the relationship and he bugged me forever after I moved out because no one at the club wanted to touch a 30 year old nerd with a drinking problem except prostitutes, who robbed his house after he passed out drunk.
Whatever side he’s on is wrong, act accordingly. He does nothing without the potential for grift or extortion. He will insist on you curtailing free speech and the rights of minorities and other fascist hogwash.