What is the ruling on picking fruit that grew from a plant that extended from a neighbor's land onto his neighbor's land and encroached upon it?

No one has the right to encroach upon his neighbor's land, not even with the branches of his tree. The owner of the tree must remove the branches overhanging into his neighbor's property, either by tying them back or cutting them. If he fails to do so, the neighbor is permitted to remove them. As for picking fruit without the owner's permission, it is not permissible. One must seek the owner's forgiveness or pay the value of what was taken, and return what was preserved.