Again, if he was employed there as an employee, there should be payroll and tax records indicating his employment. If he was employed as an independent contractor, there should be a 1099 (but it wouldn't make sense for him to be an independent contractor, as they don't receive overtime pay). So there should be a paper trail, above and beyond the checks he received.
Additionally, if he was performing work for them, even if he can't recall specific days (which is perfectly reasonable), he will be able to recall some days, and he will be able to recall general periods - and who else he saw at work on those days. Since he was purchasing hardware for his work, and was being reimbursed for that hardware, he should have records showing those purchases. (And if he doesn't, his vendors probably do.) Similarly, his own tax return should reflect the income he received.

