How does she secure the unit? I suppose if the unit is in place and the luggage gets mis-directed, it might be helpful in locating it. But if it isn't secured somehow, it could get 'disappeared' if someone gets into her luggage while in transit. I'm sure you could make a case that the device could then be used to track down the thief, but I suspect you might have a hard time actually doing that tracking down/follow-up for you.

She could secure it to the case, but I have often wondered if TSA would react in a retaliatory manner if something was secured that way. (I have thought of using a cable lock to secure contents I didn't want stolen, but I'm afraid that TSA might consider that some sort of violation and cut my cable).