What ever way you do it you will have to pay tax on it. If you need certainty on it go as a employee with her and pay the appropiate tax, use unused band from main job each week and tax rest at higher rate. Do a balancing sttaement at year end and get back tax if overpaid
I would not accept it as a cash Job, as a simple question on a Revenue Audit is who does up your books and how are they paid- If she saids she paids you cash, could end up with Audit yourself