It kind of sounds like your friend is trying to get YOU to guess what it is. A lot of the time if there is someone I want to tell I'm bipolar I wait until they bring the subject up for whatever reason.

I have a friend I am pretty sure is bipolar and her boyfriend and I have been waiting for her to realize it for years. If we brought it up her reaction would be BAD and it would send her so far into denial it wouldn't help her. I've been dying to talk to her about it but I don't.

However, if someone says "I'm really hurting, I don't know what's wrong with me, what do you think?" I think it's okay for you to tell them what you think and why, I think the difference is whether or not they are asking for help.