I had my previous A3 tinted, the peices are cut to shope and then applied to the window with a light spaying of the windows with a very very weak mix of water and liquid soap. The soap is just to ease the fitting process, the tinting film sticks to the window due to suction as the water evaporates.
To take the film off I would find a corner of the film and pick it free, try and slowly pull it so it does not tear, once you have enough of an edge, try wetting the glass side of the film with soapy water to ease the process.
If you want the car why not make it a condition of the sale that he get's it removed, or ask him where he got it done and call them for advice.
Let us know if you do remove the tint. I have tints on mine and initially i wanst a big fan, but i am starting to really like them. Suits the car pretty well. Only problem is they are pretty dark, and still takes a bit of getting used too.