Horrible encounter with WEEEED. Need help

Pre-context: My friend smoked 4 times and the first time he got caught by the police. So me and my friend were about to go to a sox game with my moms boyfriend and we wanted to smoke. So we smoke a bunch of weed (Rhino Crack (which is insane weed)) and his heart starts beating fast. Eventually after 30 minutes he had anxiety attack or whatever you call it. Our question is: If we do this in the future, how do we stop this from happening? He still wants to do it (cause its awesome) so help is appreciated!