In my 20s, I found out I had a half-sister. It turned out quite a few people must have known - grandparents, aunts, uncles. Cousins on one side assumed I had been told a limited story, but not the whole truth - I had had no idea at all, so it was all a huge shock, and I was left not knowing who I could trust. If such a major thing had been hidden from me, what else hadn't I been told?
But also, when woukd have been the right time to tell? Needed to be old enough to understand, but you don't want to tell someone something potentially confusing and upsetting when they're dealing with starting or changing school or doing exams, etc, and a lot of the time, people probably just weren't thinking about it at all. There never woukd be a right time, and then it's such a habit not to talk about it, it's easier to carry on... And because no one talked about it, no one knew what my full sister and I did or didn't know already - when my half sister told me, she said she might have approached it differently if she had realised we had never been told anything, not even a made up story with half a truth.
Try not to take it out on your friend - she was put it in a very difficult position, and this sort of thing coming out can really turn your understanding of the world and your relationships on its head. You need friends and support. Good luck with working things out.