I would suggest staying friends/in contact with him until he gets help.
As a friend, you can at least guide him down a less dangerous road than if he was left unadulterated.
Like pretending to be the deciple and talking him out of going on a killing spree or some similarly retarted event that'd f*ck up his life. Or even just convincing him that his character isn't supposed to be using drugs/robo-tripping.
If all other roads are exhausted, I'd at least try to shape his delusion into more of a hero type who helps people rather than an apocalyptic villain. (With the help of the other deciples or whatever.)
Random acts of kindness > Random acts of violence.
I'm not sure if that'd feed into his delusion and make things worse though, I'm no doctor, but eventually when he snapped out of it (if he ever does) I think he might thank you.
Worst case scenario, he decides you've went Judas on him and he tries to kill you - which is when you kick his *** and show him he's not as powerful as he thinks. If there's one thing I know for sure, these type of people spend much more time stroking their ego than they do actually getting stronger.
Although if you do fight him, fight to take him out; odds are he'll be doing the same.
Ionno I guess I'm just rambling.