What I've tried so far:
* Checked the screen timeout in settings and put it right down to 15 seconds
* turned phone off, left it for a while, then turned back on
* taken the battery out, left it for a while then replaced and turned back on
* tried killing apps in "Running Services" one by one and none of them seem to be the culprit
* Uninstalled the only app she's installed recently (some facebook messenger app)
But it still does it. Any more suggestions to try?