I always ask them if they deposit any payments on their bank account, if they do, start there.  If they said they got cash, ask if the employer can write them a letter saying that he paid them x amount of money.  If they don't believe that the employer will do that, then have write and sign an statement of how much they made with as much detail as possible.  Depending on the kind of job they do, probably they will have expenses and ask them for receipts of expenses.  If they cannot provide that then is better not file taxes for them.  My opinion.