Have him look at the hand-feed tray. If there is a paper width
adjustment, see how narrow it can be set. If not, there would be no way
to guide a narrow envelope correctly into the printer.
My HP 4600N hand-feed tray width can be set for less than 3 inches (7.6
cm). It has an image of an envelope embossed nearby on the plastic,
that shows the correct orientation of an envelope to be printed.
The HP 4600N prints to a Number 10 (Business-size) envelope (4 1/8" X 9
1/2", 10.5 cm X 24.1 cm). There is an option in the computer's page
setup for a "#10 Envelope".