Idiot-proof is one thing. Getting someone to commit to doing things when they say they will is another. This is what I'm going to guess is what happened.Man, that's awful! You definitely have to make it idiot proof when you go away for more than a few days if someone's going to care for it. What the heck happened???
I just came back recently from a week trip out of the country. My in-laws (who speak NO english) fertilized my three tanks and feed the fish for me while I was gone. When I came back ALL was perfect. As if I never left, only bigger plants!
The way I did it was I bought 6 different daily pill dosers from the drug store for about $1/each. Using a sharpie marker I labeled each one with a Chinese character for big, little and medium (for the tank sizes) and added one character for food on the ones to be used for the fish food. Then in each of the SMTWTFS compartments I put in the food for one day in the food "doser" and either dry macro or micro ferts in the fert dosers (I use the EI method). The key was them coming in at least once a day and opening that day's compartment of the right doser and dumping it into the right tank. They commited to that, and they did that.
I'm guessing even with simple, easy instructions what we had here was likely someone who forgot some or even all days!