If she is dependent on her husband, courts will award her alimony and all kind of other supports.

Moving out to live with another man might jeopardize her getting half of the house and possibly spousal support.

Of course if she is in danger, it’s better to move out in secret but she’d benefit from consulting domestic abuse shelters as well as a lawyer first. Moving in with affair partner without consulting anyone might not be best course of action.

You planning on getting married soon will not work as she’s already married. It’s not legal.