If the T8 isn't set to watch one episode after another of the same TV series, then the internet usage shouldn't be that high at all. If they were watching a live stream of a TV channel, that would carry on forever. Hmm, I'm starting to see your point more as I type this out!
Maybe build up to turning off the device, see if they're happy to press the Stop button on their remote as a gentle introduction to such procedures!