..I had a similar thing happen to me once - got to campsite and found a flat tire, called Good Same ERS.
Within an hour a guy from Wingfoot Tire service (a Goodyear dealer) comes out. He took tire off, dismounted it from wheel, thoroughly inspected it inside and out to be sure was OK, repaired issue with valve stem that caused the leak, inflated tire back up, put it back on, all is good. Had brought a tire with him of same size, brand I already had, would have sold such to me and put it on if was needed (fortunately it was not).
Since he HAD repaired the tire (valve stem), I was handed a bill for a whopping $15. I thought that was very fair and gladly paid it, especially for as remote as we were and how quickly they came out.
All that said, I do agree that if all that was done was to replace the tire with the spare your friend already had....There should not have been a labor charge. If they had to do anything to REPAIR a tire, that's a different story.
I see GS is involved with this thread, hopefully they will make it right for you.