I would just hard code the flag in the fedex include file.
They probably want to update the recipient address... that is on line 120. You can change:
$FedEx_XML .= "<v10:Residential>0</v10:Residential>";
$FedEx_XML .= "<v10:Residential>1</v10:Residential>";
There are actually three places that could be updated. The Shipper Address, the Recipient Address, and the Origin Address. They are on lines 106, 120 and 134 respectively and can be updated similarly where necessary.