Yaris alternator problem

[caillou]

My sis bought a 2007 Yaris. Drove for 5 days - no problem. Then it died. Boosted up - started easily. Next day it died again. Changed the battery, drove for 10 days. Died again. Toyota Dealer diagnosed - dead alternator. No light indicator ever came on to tell there was a malfunction. All light indicators worked properly as they should have. She towed the car to the mechanic of the dealer who sold the car and promised to replace the alternator for free. However, his diagnosis did not reveal any problem with the alternator or any other problem. He just boosted the battery and the car is still running. My sister called the Toyota dealer mechanic to report a "no problem" issue who said that it's got to be the alternator because they've encountered this problem before, and the test may not necessarily show the problem (????).
Anybody? Any idea? Should she be driving it untill it stalls again?
I tried to ask this question on the ToyotaNation sponsored "JustAnswer"(Ask a mechanic) - you can see it in the blue box on this screen, but they charge $23 for the answer. I wouldn't mind $23 provided the answer is helpful but how do I know?

Thanks for any help.