Wow, glad everyone appears to be ok, but this scares me because we are about to go on a two week vacation to Wyoming next month. I fully understand you being ticked off! I personally would have expected a call from her after it happened. That could have fixed everything and it wouldn't have been a big problem in my eyes. You could have talked her over how stuff was supposed to be hooked up and what temps/etc should be so that you know everything was working properly. Instead it sounds like she just tried to cover it up or ignore the fact that it happened. I know when we leave we will explain everything to my mother in law who will be caring for critters while we're gone and show her how everything works. Luckily she has a good head on her shoulders. I will also have a friend, who keeps many reptiles, come check on them periodically.
I like kitedemon's lock idea and it would work great for keeping out kiddos, but I don't like the idea of no one having access in an emergency.