It really depends on the person but from I had a good experience with a friend I knew was BP when I saw an opportunity to broach the subject.

He was having a rant in a manic manner that I recognised as such, so I chose to take a risk and talk in a manner that built up to the fact that I was BP. He initially seemed shocked and then I think he saw on my face that I knew he was BP too and it was fine.

He wouldn't stop talking about things for a few hours then.

That being said, it was clear to me he knew he was BP and was bursting to say it out loud even though he felt ashamed. It may/will differ for other people.