Oregon is 6 mature, 18 immature plants for medicinal use. I know California doesn't have per-plat law, but that seems like a lot.How do you know that? Pot is like tomatoes. Nobody that has a garden has just one tomato plant. In certain states he may not even be violating any laws.
If he gets a "knock and talk" from the local authority and shows proper documentation, then what's the big deal?
I would rather someone check and be sure than assume he's legit.