Old Dude

A friend of mine has a 2012 Q5. The pedal went to the floor the other day and she had to pump the pedal to stop. It was fine when she restarted, so she didn't go to the dealer. Last week it failed again. I was there and verified it. There was a strong smell of burning brake fluid at the front left wheel. No leaks. The dealership has it and cannot replicate it. They saw minor scuffing like a rock got between the caliper and rotor. Everything works fine. Pads and rotors were replaced 6 months ago. Still under warranty. They have driven it over 500 miles. Says she needs to return the loaner. She is scared to drive it. Any ideas?