There was a miscommunication about the state of the vehicle: I did not inform Paige until the time of the rental that the "check engine" light was on due to a bad glow plug. I'd had the car checked by my mechanic and was assured it was serviceable. Upon nearing her destination, Paige experienced a problem with the car not shifting into higher gears as expected. She contacted RelayRides roadside assistance who contacted me and eventually turned the car off and on which cleared the condition. I attempted to contact Paige directly but could not get through to her. After the vehicle was returned, I had my mechanic check the car upon replacing the faulty glow plug and he assured me the car is in good working order. It's possible that she had triggered "sport shifting" mode which raises the transmission shift set-points to a higher RPM which persists until the car is turned off. This behaviour is documented in the manual which I keep in the glove compartment, but I was unaware of that function as I don't usually drive my Jetta very hard and hadn't experienced it myself.
Before speaking with me on the matter, she filed a review which stated an assumption of malicious intent on my part. Based primarily on the poor communication of this situation during the trip and secondarily on her assumption that I would knowingly rent out a car which was not safe to drive, I would not rent any of my vehicles to Paige in the future.