What is the ruling on an accountant's work in settling taxes for entities whose operations are questionable, such as banks, hotels, and gambling halls?

Working in accounting is permissible in principle, as it is a beneficial science that leads to benefits, provided that accounting is a means to something permissible in Islamic law. Therefore, it is not permissible for your friend to work in it if it is a means to something forbidden in Islamic law, such as assisting institutions that engage in forbidden matters, as Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."