I'm always using that on roaming data! Even then, Google Maps happily blows through hundreds of megabytes of photos within a couple of minutes of browsing for restaurants, probably for all the high-res photos in the reviews...

On iPhone you can set Low Data mode for all cellular data (and also for specific wifi networks).

She might have kept tapping Sync at the bottom of Photos even though iOS itself pauses it when in Low Data mode. iCloud Photos and video syncing is a data killer if you're on a holiday abroad, my wife takes hundreds of photos and videos a day, imagine what it does to data.