Activist Headquarters

... if possible. Put yourself in their position: would *you* like someone coming on your land?

If they found out, the best your family member could hope for would be to have all their work destroyed.

Now, if the property is abandoned it may be that the city (...or whereever...) may be interested in neighbors providing upkeep for free. I have lived in neighborhoods where we mowed the lawn of an abandoned house, and if a few spare bulbs found their way into a garden, who's to know?

Planting food crops is another matter. They take more tending, which means more trespassing. Is that really a good idea?

If your family member's interest is in raising food cheaply, contact your local "sustainability" group; they may well have resources for things like this, especially if the land is abandoned. Producing local food is something to encourage and often city council members (or whatever) are inclined to respond to voter concerns; unused public land in particular might be turned into a "P-Patch" if your family member is willing to do the work.

This *does* take the "guerilla" out of "guerilla gardening" but you should remember that the usual fate of a guerilla warrior is unpleasant. Sorry to be a downer.