You write her a mail stating all of the above, that you agreed to help her fill her taxes. You prepared it correctly according to your knowledge, and that you advise her to declare the bonus received.

If she doesn't, she would take over against your advice and you would stop assisting her, as it would be a false declaration of income.

She doesn't have to answer, you just need to keep this mail very carefully, as well as evidence of the prepared tax you did, including the date.

Think of it as if you were an accountant with a mission to prepare your taxes, and your client wants to take over against your advice. That would be a breach of contract and you would not be responsible anymore, as from the date you notify your client that their action caused said breach of contract.