Be it a pruned or full synchronization you still need a decent amount of space and stable power supply since you can't risk your computer turning off during the syncing process or else sometimes you could restart since the files could get corrupt.
Correct, if the computer he is using looses power all of a sudden, his progress would not be saved properly and the track of new blocks synchronized may be lost prompting him to start again from the last point that got properly saved. It would be safer for him to hibernate his PC when he senses possibility of power loss or he can close the software so that it properly saves his progress before shutting down.
How exactly did you set up Bitcoin Core to prune? You might have entered a large size. If you don't want to consume more mobile data, then try lowering them to "550MiB" which is a minimum, or just set it to "2000MiB".
This doesn't make sense. Pruning doesn't mean you have to download less.
Yeah, sure it only helps save disk space by keeping only recent block data and deleting older ones that your node no longer needs but you would still need to sync the remaining required blocks to be up to date, so there is no way you are downloading less than required.