Does the work of a young accountant with his father, who owns a cigarette factory, invalidate his righteous deeds such as prayer and fasting?

The means take the rulings of the ends. Therefore, anything that leads to the unlawful is itself unlawful. Smoking is forbidden, and what is forbidden to consume is forbidden to sell or manufacture. Hence, the manufacture and trade of cigarettes are forbidden. It is not permissible to work in a factory that produces cigarettes, even as an accountant, due to the assistance it provides in manufacturing and promoting something unlawful. You must advise your father to fear Allah and replace the tobacco industry with beneficial products, explaining that Allah's provision is not attained through disobedience. Working in the factory does not nullify other good deeds, but it brings forth sins that might outweigh good deeds and lead to one's destruction.