Tom clearly needs help, but that doesn't mean you have to be the one to give it to him. This story just sounds all around awful. I hope he gets the help he needs, and I hope you know there was not a damn thing you did to deserve this. Drugs are powerful and effect people I'm lots of ways, they become altogether different people. 

If his family doesn't already know,  I would inform them so they can tru to get him help. As for you I would move on,  cut the friends out of your life who knew, and don't stress thinking that you did anything wrong.

Unless he was just smoking weed. All you mentioned were drugs and high. If that's the case then fuck Tom and his weak ass excuses for infidelity. 

(Also want to clarify he's an asshole regardless, but a much bigger one if this was something as simple as marijuana.)