is it cable or vacuum operated.
you need to remove the cable or vacuum pod to check it.
if it is still sticky i have used silicone spray. it might smell a bit when it gets hot and i did not read the label regarding how many types of cancer you can get it you breathe the fumes.
if it has vacuum pods you need to get a small hand vacuum pump and check all the pods. if they leak you can but new ones.
check the vacuum can in the engine bay.
check the white plastic valve in the front of the heater box it it has one. it is closed one way and should leak vacuum the other way.
you need to remove the cable or vacuum pod to check it.
if it is still sticky i have used silicone spray. it might smell a bit when it gets hot and i did not read the label regarding how many types of cancer you can get it you breathe the fumes.
if it has vacuum pods you need to get a small hand vacuum pump and check all the pods. if they leak you can but new ones.
check the vacuum can in the engine bay.
check the white plastic valve in the front of the heater box it it has one. it is closed one way and should leak vacuum the other way.
There are a couple of things that can cause that to happen. The most common is that hinge of that door has gotten rusty or has an accumulation of crud that makes it get stuck in one position. Or, it could be that door got a little bent somehow. I would first try disconnecting the cable, spray the hinge with some PB Blaster, and swing it back and forth a bunch, and repeat that process a whole bunch of times to see if that freed it up. Since you are the one that has the car sitting right in front of him, you'll need to move the door very slowly and watch to see what it is getting stuck on. 
