Blimey April, how horrendous for you.
Nothing wrong with your boss telling the team as such, but obviously lots wrong with telling you when others don't know and asking you to keep it a secret.
Hmmm.
Whether she should have disclosed it when she started depends what the norm is when starting at your company, whether there's a medical questionnaire requiring you to state anything like this. Do you know whether there is anything like that?
You say this is affecting her performance. Has it done so for the whole time she has been there? I ask because regardless of whether she wants to tell her manager, if it's affecting her performance, I would assume hope it would have been picked up in her appraisals or outside that process, and I would say a medical condition would be a better reason for her to give for not doing her job properly than just being rubbish iyswim.
I can only assume either her performance is not being affected sufficiently for her manager to notice and pick her up on it, or alternatively you and the rest of the team are covering up for her.
What kind of relationship do you have with her? Is it the kind where you are quite friendly? Do you feel you could go out for lunch or a drink with her and talk about this, say it is making the team feel uncomfortable?
Has she told you why she doesn't want anyone to know?