Key here is "probable", make them prove it (still under the 3/36 ?). Find the leak (should have oil around it). A rock should have bent several fins around the leak besides making a hole in the fascia (must get through the fascia and fins to the tube) & be obvious. Might want to get a 2nd opinion from a local shop or another dealer. Did they put it in writing ?
BTW the condenser is in front of the radiator, evaporator is the dash, could see a lot of labor for the evaporator but no way a rock could get there.
ps look here