All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If all the money of the employer is forbidden, then it is not permissible for you to work for him, neither as a tax advisor nor in any other job.
But if some of his money is mixed, some of it is ill-gotten and some of it is lawful, then it is permissible for you to work for him if his work is in permissible trade or industry.
However, you should not help him to evade the taxes except in the case where this is permissible, as we have explained in Fatwa 203644.
Allah knows best.