Problem is, he has used her. And he’s a grade A cunt,
If she was vulnerable then it’s going to be a bitter pill to swallow, and she’s not going to get much sympathy.
But I would tell her this:
It will only hurt her, it will not go the way she wants, she might get some instant relief (and I beleive she probably wants to hurt him, not the wife, but currently that’s the only way she can hurt him)
But she won’t hurt him, people like this don’t get hurt in the same way she’s hurting. And it will eat away at her, before she does it, and if she does it, afterwards.
The only thing she can do, is get to a therapist ASAP. Try and work out why she ended up with someone that cruelly manipulated her for 2 years, and why she was happy to only take the crumbs for that time.
Then go on a live a happy life.
I would think it’s very unlikely that his wife doesn’t know something already.