When she cleaned it how much water did she remove. If she removed to much it can be from that. Fish tanks go through a cycle and if to much water is removed in losses all the good bacteria and the cycle process has to begain over. Some people I know add that cycle chemical to the water and feel it ok to remove more water then normal this way, but that can be iffy.

It can also just be from stress, if the fish were stress out in the water changing process, it could kill them. The water could have been to cold/hot, if the changing took to long and they were with out oxygen in the tank for to long, sometimes just the stress of just going through a water change can do it, depending on how it was done. Moving the tank could have stressed them. Stress is the #1 killer of fish.

With out knowing what kind of fish they were and what she did in more detail, it really hard to tell. So many things could have happed. Did she add any new things to the tank? Did she add any chemicals to the tank? If so was it to much or to little? How long did thet go without filtration? You see what I'm saying, there just so many things that can go wrong and harm a fish.

I'm sorry she lost them though and sorry I could not be much help.