On a karmic side look at it like this.
People should form a relationship with the plants essence rather than just view it as an object to be used for your own pleasure. So, as natural course of action when you had your hands upon it, in it and you were involved in the process of bringing it to another form you initiated a relationship with it. You put some of you into it, and some of it attatched to you. You both had expectation of furthering this relationship.
Once he decided to let greed rule and ignore this relationship he alone disrespected the plant and relationship and opened himself up for a teaching opportunity to set him right. You can take some comfort in the fact that the universe did not ignore this happening no matter how small and insignifigant it may seem in the big picture. Its all part of the web and the small disruption in balance will be corrected sometime.
Dont begrudge him and dont try to be karma master and do a payback.
He likely didnt do it just to be an asshole. Help him learn and let it go.
If he accepts the lesson , all is well. If not the universe will find a way to teach him.
Its all good in the end.

If I sound preachy it is because I have a different view of Marijuana than many of you, especially the younger crowd. Too many of you react with violent ideas and revenge and the like. I try to show a different perspective than some of you might think of . Its all about respect.

Peace.