What To Do? An Ethical Dilemma

I saw an interesting question in a sefer on Shabbos. A psychologist was told by his patient that she no longer goes to mikva or keeps kosher - without her husband's knowledge. Must he tell the husband or not?

On one hand he must maintain confidentiality while on the other hand the husband is transgressing issurei kares every time he is with her and eating from unkosher keilim in his own home and there is a mitzva to separate a person from sin. She refuses to tell her husband under any circumstances.

Big problem....