Is it possible to burn out/mess up your transmission/clutch on a manual transmission car by always holding in the clutch at a stop light?

A clutch should last 100000 miles. The driver needs to get a driving instructor to see what she is doing.

Modern clutches are fine to keep fully down at lights. Old cars had carbon bearings that used to fail.

Yes you can burn the centre plate of a clutch. This is why in Europe we have a different license and test to drive a manual.

My guess is she is slipping it in traffic you can smell a burning clutch. A few proper lessons are cheaper than new clutches.

The part that suffers is the throwout bearing, although the pilot bearing also lives a lifetime in a few months as well. In normal operation neither the throwout bearing nor the pilot bearing does anything. The pilot bearing, in the center of the flywheel, just acts as a centering device as long as the clutch is not depressed. The throwout bearing is doing even less, just waiting for the clutch to be pressed.

Holding the clutch in while waiting for a light can increase wear on those two parts by many hundreds of times. In that sense she is only burning out components of the clutch, but it is silly to replace only parts of the clutch after going to all the work to get to it.

That wears the throwout bearing, not the clutch disc. I agree with you that she's probably riding the clutch pedal. That will wear out a clutch for sure.

3 clutches in 5 yrs should have answered your question for you. pay no attention to them!

no