When water evaporates out of the tank the minerals still stay in. So when you replenish the water with dechlorinated tap, you just keep on adding more and more minerals in that will never go away if you don't do a water change.
i agree that if you have limited fish inthat tank maybe you only need minimal water changes once a month is not that detrimental ...but you don't let them go for four years ..which is what she has done...to me thats a little wrong not only is it unhealthy for the fish but it starts to smell and its just not clean or even good looking becuse of the lime or calcium build up...
Emg,
It is possible to find stasis in a tank but it is never preferable. You can always improve on the conditions. Eventually they will fail if left alone. The environment is just too small. Many people succumb to the belief that just adding water to compensate for evaporation is fine. They keep fish for years this way. They just don't realize they could have been doing a better job and in the end the fish would have been better served.
If you have 10ppm of whatever and you add water due to evaporation, all you do is reduce it temporally. By the next week you will have more. It will just continue to creep up. Eventually it will be too much.