I have a few friends in my life who have seen me manic, depressed, etc over the years. At this point we’ve largely fallen out of touch and I’m better handling my illness but I still want to tell them. I’m not sure why, maybe just to spread awareness I suppose.
When it feels right, so probably now. I think you’ll be surprised at people’s reactions to hear from you. Even if your friendship ended on a bad note, most will still be happy to hear from you and catch up.