That really is sad, and unfair, but unfortunately very common. So many people take help and expertise for granted, without any consideration of that it should be reciprocal. Even more, when you don't want any credit for your work, but just see the plants flourish and become beautiful - and their negligence and resistance against good advice puts even that in jeopardy.
I've been in that position many times, and now I carefully select when I help, and when not.
I will do that going forward, it was a lesson learnt.
I hope that works!