Working in an accounting firm

A person works in an accounting firm in USA where his main job is to do book keeping and monthly sales tax returns. He also does personal income tax and business income tax yearly. A situation arises where his boss ask him to fill out a client's previous three years business income tax based off estimation. The revenue will be reported accurately, however expenses will all be estimated to show a loss for the past three years. Is this permissible to do or no? Secondly, is it permissible to intentionally exaggerate expenses to reduce the amount of tax that will be paid to the government? Lastly, will it be permissible to estimate certain income and expense figures that are not available? Pertaining to the last question, even the best accounting system does require some level of estimation. First question is regarding the permissibility of the action. Second is that if this is 20% of his job, is his income halaal? Some level of detail will be greatly appreciated. JazakAllah for your answer.