I don't have any background on this case specifically but i know a lot about 'manual drivetrain' in general. This sounds like one of those problems that stems from lack of use. As in, if that shifter doesn't get moved for a long time, it becomes hard to move. 4wd transfer case shifters in trucks frequently deal with that because of lack of use. Most of these types of 'power transmissions' with manual shift levers, are splash lubed. Sometimes that area gets NO splash lube if it isn't engaged and spinning, and sometimes it's just that the surface between two part of the shift mechanism spends so much time 'covered up' from the splash lube while shifted in that position, that it 'dries out' and starts to rust just from the moisture in the air. Shifting the thing back and forth and operating a little while in each position gets all those surfaces coated in oil again. As long as oil gets splashed onto them every once in a while, they will basically never rust. This is part of why the concept of 'ran when parked' is such a joke, because you can pull off the cover of a low-hours transmission that's been sitting still for years and years and everything above the fluid level is rusted. Same thing with gas tanks. If it was parked empty the exposed tank will rust out. If it was parked while full, the gas itself may be beyond bad, but the tanks walls didn't rust.
Getting it unstuck is one thing, but once it's unstuck the simplest long term solution is to move it every once in a while even if you don't have to.
Hopefully the repair bill is reasonable!
Semi-related: I have done some real funny looking stuff trying to unstick clutch discs from flywheels on cars that have sat for years and years..
