Is the questioner considered sinful and is expiation (kaffarah) binding upon her for causing the loss of income for the lawyer to whom she offered the bribe?

What the lawyer presented is a betrayal of her employer. The primary step is to conceal the Muslim's fault, first by advising her and clarifying the impermissibility of her action. If she insists on her betrayal, then it becomes permissible to inform the company about her, out of sincere advice. A servant's provision is in the hands of Allah; no one can cut it off.