Engine high idle

This is my girlfriend's car. She said that late last week (Thursday or Friday) she was driving along and pushed the gas pedal to pass someone, but when she let her foot off the gas, the engine kept racing. She had to shift into neutral and kill the engine to stop it. After she re-started the car, it was back to normal, and the problem hasn't surfaced since then. She had it examined by a mechanic (not a dealership), and this is her description of what he told her: He said she had a bad temperature sensor, but in order to replace that, he also had to replace a "gasket", and "something else related to the engine". She questioned him further, and he told her that if he didn't replace the gasket and the other thing that she could end up with coolant leaking into her engine. The total comes to about $800. I have three questions: 1) Can a temperature sensor account for uncontrolled accelleration, 2) what parts do you think he was talking about, and 3) does $800 sound reasonable? Thanks for your advice.