OP's heatpump model is the same as mine...The Galley escutcheon plate has both the return air and a "Quick-dump" supply Diffuser, side-by-side and directly below the rooftop unit. Here's some quick items to check:
1. I would inspect this area to see if there is any supply air "bleeding-through to the return-air side" ; thereby not even getting a chance to feed the supply duct. Remove the filter assembly and see if you have a leak from the supply.
2. I've seen some forum reports of a expanded foam gasket seal dislodging and moving into the ductwork. Remove the escutcheon frame from the ceiling & do a hands-on inspection.
3.If you have excellent air flow at the dump grille; the distribution duct is the next item. I would shine a light down-the-length of duct and visually see if there is some sort of blockage. Your model has 2 rows of supply grilles. If one side is good & the other side slow? or, the entire length, fore and aft appear slow?
4. If the OP is not the original owner, the previous owner may have inserted something into the duct. Many-a-mod has confounded 2nd owners.
5. Your friend may have a very different unit than yours; but I believe the air flow in cubic feet per minute would not vary [not related to the BTUH capacity of the unit]. If he had 2 or more units running in unison, you would have noted that already.