I'm not at all sure the premise for your question. I am also not clear if you are discussing an individual, corporation, or other entity. But, if you are wondering if you have to provide documents with a tax return, then the answer is generally "no." Forms W-2 are the only documents sometimes required to be filed with an income tax return for a U.S. individual.
A tax return is not about proving income, but reporting it. The Internal Revenue Code of 1986, as amended, requires taxpayers to report all income from whatever source derived, unless specifically excluded. From that, taxpayers may take deductions, credits and other adjustments, as allowed under the Code.
The Internal Revenue Code is, essentially, an honor system. But, of course, there are audits. So, your client should strictly adhere to the Code in reporting income.