You should tell her. I can't understand why a parent wouldn't tell a kid about the kid's life, when it could be just a matter of fact thing.

If the parent doesn't tell by a certain date, you should tell the kid. Trust me, if the kid is lied to, they won't trust the birth parent, the family friend, you, or anyone else in this world, especially those who knew and didn't tell the primary person involved, the kid.

Trust me, I'm the poster child of not knowing until my 50s. And now I'm faced with never trusting anyone ever again. Tell the adoptee!