How would you handle this public "toker"?

Here's the situation (and before the low IQ's chime in, doing nothing is not an option). An employee at a business just down the street from my home takes regular weed breaks during the day at the same outdoor spot under a tree - away from the office but still on-premise and in full public view. I've decided to leave the guy an anonymous note letting him know that I know and give him a chance to stop on his own.

What should the note say? I'm assuming it's not a prescribed/medicinal user. Even if it is, he needs to be more discreet.