... depending on how he grows it, someone saw it -- through a window, etc.

Or someone mentioned it to a friend, who mentioned it to a friend, who mentioned it to a friend... Remember the old Kevin Bacon game. It doesn't take many hops to go from "trusted buddy" to "The Godfather".

Or someone hacked the list, knowing it would provide a group of people who are growing weed but don't take any of the precautions that the usual crooks doing so would.