After being locked out of his Samsung tablet (supposedly it set itself a lockscreen out of the blue), I checked whether it had backed his photos up on Google Photos, but nothing there... After resetting it (it seems Samsung removed the ability to reset the lockscreen password via your Google account) I assumed that the photos were lost. However upon opening the app we rejoiced when they started appearing. Shortly afterwards, the Google Photos app popped up a message stating that an upgrade was required, after which all of the photos had disappeared again.
The workaround was to reset the tablet, open up Google Photos, wait until the photos had synced and then disconnect from the internet as soon as possible to prevent Google Photos from trying to update itself (the message couldn't be dismissed). My hunch was that the version of Google Photos that shipped with the tablet was very old and they have long-since updated the format for storing photos, hence why they wouldn't show up online?