Wash the tomatoes when they pick them? I just don’t know if this would impact the tomatoes other than actually killing them and the plant.

If he did do this, then I think it depends on intent. Was he trying to hurt them or just kill something of theirs, like the tomato plant? If he was purposely trying to hurt them and that can be proven, then I’m guessing it could lead to some sort of felony assault. If not, likely a misdemeanor at most.