Process the invoice for 5 units, charge the difference to the inventory adjustment account so that you pay the full invoice. the debit to the inventory adjustment account will offset the physical inventory adjustment Fix the PO by posting a ROG for that item of zero quantity and backorder zero (assuming there is nothing left to receive.
Then take employee to the woodshed to review the SOP for not making mistakes. Rinse repeat if necessary. If after the third time, mistakes continue and remedial action is not working, find someone who is less mistake prone...