I have a hard time believing that an ecu would go bad, but the car would still run, I'd go the autozone route and find out exactly what is wrong with the car. It sounds to me like they are just trying to get her to trade it in. I can imagine if you went the go ahead and fix it route they would replace the ecu, and some cheap sensor that was actually the problem.
2500K@4632.1, 7970 Crossfire, 2x Crucial 64gb raid-0, 240GB Steam Drive, 16gb Gskill
CCNP, Working on CCVP(1/5)