Do I understand you to say that your parents have a garden but it is getting out of control and a friend of theirs wrote to you to tell you and suggest you come and tend it?
It is hard to comment without reading the letter, however, yes we do have a right to set personal boundaries.
However, if they are concerned for your parents and the condition they might be warning you of something they feel you have not noticed. Are there other siblings? If not, in a way you are the one to arrange help for your parents. Perhaps this person is suggesting it is time you stepped in as things seem to have gotten beyond their control. Perhaps changes need to be made to the garden or hired help needs arranging.
If the letter was mean spirited then yes you should not get involved in a toxic relationship with the complainer.