That makes more sense. Maybe he/she/both receive assistance(or enrollment status) from some source that would eliminate them if either of them lived in a place with $885 rent (for example, they were only allowed to spend certain % of income on rent). I have tenants with household members who receive disability payments and they sometimes request receipts or a copy of the contract that shows the rent amount.
I think I'd be willing to oblige, even if I suspected them of gaming some outside system, so long as the receipts were accurate and matched the total amount paid. Of course it would be silly to issue him one saying he paid $885, and another to show that the mom paid $400. But one for $885 with an itemized breakdown, or two for $400, $485 seems fair enough since it is accurate.