Have you googled anything to look at a wiki or youtube explaining an AC system?
I wont get into details- 20 minutes of typing or a 45 second video. Give it a look.
The compressor moves freon between the condenser (by the radiator) and evap (in the dash). I do not think a failed comp can cause the evap to LEAK. I do not think working on the comp can damage the evap. I do not think that debris from doing a sloppy job would cause an evap leak. (note these are all 'dont thinks'- im not an AC tech)
How did they determine, conclusively, it is the evap? Using a freon detector and finding freon blowing out of the dash?
https://www.youtube.com/watch?v=kwjcghZ3b8s
https://repairpal.com/estimator/bmw/...placement-cost
I cannot find a price on your part, but 200-300 seems possible? Id be OK with a behr replacement, if one exists
Based on the costs listed at repair pal, I would get another opinion/estimate.
I would do the following:
1. Determine how precisely they diagnoses the leak in the evap. Id demand this in writing.
2. Find a shop that will quote and AC evap replacement.
3. Once you chose a shop, ask that they confirm the diagnosis; determine of the prior work caused this failre; ...then probably do the evap replacement.
Thats all I got...