Has your friend tried to talk to you about anything yet? Does he know you know he's bipolar? Maybe if he knows you are a loving and non-judgmental friend (which I know you are) he will be able to share with you, or at least be able to turn to you when he's doing badly. The thing though is that when I'm personally in a bad phase, as much as other people reach out, I still feel completely isolated and alone. Its this thing in my brain that just doesn't let me connect to anyone. Usually it is agonizing. The best thing is when my husband just hugs me and lets me freak out. I know its not much, but him just listening (even though he really doesn't understand) is way better than suffering alone. Continue being the kind friend that you are and make sure he knows you are waiting for him when he needs you.