I want to look really closely at the question of why the secrecy was necessary. As other people above said, he could have been vague but reassuring; he could have asked the person he was helping for permission to bring his wife in; he could have gone above and beyond to reassure her she was still loved; and it doesn't look like he did any of the above. Was he getting off on keeping secrets, or on the cousin being completely dependent on him alone? (In which case--- feeling like she was being cheated on is maybe not 100% inaccurate.) Did he not trust her to keep it secret from the people who couldn't know? In that case there's bigger problems in this relationship.

But also - the question she's actually asking is "I know we need couples therapy, but before I go, am I in the right or is he?"

And the answer to that is - knowing who was right and who was wrong isn't what you should be looking for in couples therapy. You will need to get past that if you want the therapy to help or the rift to heal.