As many other people have mentioned marijuana affects everyone (bipolar or not) differently. For me it makes me anxious and sad. For my best friend it relaxes her and relieves her of anxiety. For another friend it's just a fun way to relax. For a third friend it does make them psychotic so they have to stay away from it.

In terms of your situation, I'd say leave it up to your friend and try to stay out of it, unless you notice a pattern between psychotic episodes and smoking. Then maybe bring it up gently, asking if your friend thinks they're connected or not.