She was thrown in at the deep end.
To be thrown in at the deep end means to be given a difficult new job or task without any preperation.
The expression refers to the deep end of a swimming pool.
If you were thrown in at the deep end of the pool you would have to swim or you would sink.
Example: My dad started his new job this week. He doesn't mind being thrown in at th deep end. He says, you have to learn fast and show his boss he can do it.