Tell your boss you don't want to fuck a married man. You won't tell anyone, but you don't want anything to do with it anymore.
However, keeping the job will be more trouble then it's worth for your friend. While I understand the need for income, her work situation will increase in drama no matter what from this situation. There will never be a return to the normalcy before, it's too late for that. Either she should accept a drama-filled workplace, or find work elsewhere.