I would like to know -
My friend shared her Steam library with me. Does she also have access to my games or do I specifically have to share my Steam Library with her?
One person sharing does not automatically cause the reverse. You have to choose to share your own games on the specific computer. Sharing on one computer you have logged in will not share on all the computers you have used.
As a reminder you can't both play games from the same library at the same time, regardless of sharing settings. But if you are both sharing, you could both be playing a game from each other's library.