I'd suggest a more proportionate response. I don't rule out the possibility that he's actually cheating, but it seems more likely that he's enjoying the attention without actually acting on it.

But who knows? Step #1 is to get the information on the table by actually discussing the issue, not just telling her fiance to cut the woman off.

To borrow your sense of proportion, if she doesn't start communcating better, she will die a lonely spinster with 17 cats, wondering where it all went wrong.