I dont understand how they would be likely to break it, if they knew what they were doing. I guess they are just covering themselves.
I am assuming for an 84 sud, there is just a rubber seal holding the glass and there is no trim inserted into the rubber like the early cars?
For just the rubber seals: take a couple (or more) old flathead screwdrivers (reasonably wide) or some cheapies and round off the edges, and use them on the inside of the car to 'tuck' the inner lip of the seal up under the flange of the window opening - be careful not to pierce the rubber, its pretty tough.
As you go along the seal, you may need to use some other screw drivers or other to hold the rubber to stop it springing back. Start at the top in a corner (I have found the rubber tucks more easily there) and work your way around and down the sides. When you only have the bottom of the seal holding it in, you should be able to gently push the glass outwards from the top - get someone (or two) to help you hold it steady.
If you have any mastic around the seal it may be more difficult, but usually they are dry or the sealant is very dry itself making little difference.
Its quite straight forwards, just take your time. Getting it back in, with sticky black sealant everywhere is the fun part!!