Sounds like he got a better bag of cannabis the second time around and didn't realize he should "test the waters" with every bag....rather than treat them all the same. He probably got really high, got scared...and from there a panic/anxiety attack can follow easily.

Three months smoking regular cannabis and then smoking just as much high-grade cannabis....well that is going to be an intense transition.

Tell him that everytime he gets a new bag, he needs to start out by taking 1-2 hits and seeing how it goes from there. This allows him to find his own dosage.

Also, if you haven't found it. Here is a link that will have a lot of valuable information for both you and your patient.

http://www.erowid.org/plants/cannabis/cannabis.shtml

And make sure your patient knows that using a psychoactive chemical in a setting in which you're not fully comfortable can raise the risk of having such an episode.