ditto - tell her before everyone else. Will give her time to deal with it in her own way. If you tell her along with everyone else - she will either be upset (cry), and walk off to get her own time, which if it was me, i would then feel foolish and regret spoiling your moment, or, B - will feel like she has to fake it.
Dont tell everyone first, without her, otherwise, she will feel left out, and everyone else will 'stop talking' when she walks into the room, which will make things worse.