I'd ask myself very seriously if I wanted to go into business with somebody who was prepared to break the law and not pay tax. If she's prepared to be dishonest in that respect, what guarantee have you that she wouldn't also be prepared to cheat on you financially if the need or opportunity arose? If you're planning to keep any joint accounts, then you will both be equally responsible for payments. If you're just working together but each responsible for taking payments from your separate individual clients, then your tax arrangements will be separate, but you're still putting yourself in a vulnerable position being in business with somebody who you know is breaking the law.