The enhanced Fidelius didn't erase her completely. Unlike buildings she could still be seen, heard, talked to.
But, unless Cedric introduced her, they forgot her existence once she stepped out of sight.
What remained was a memory of a conversation where they weren't quite sure who they'd been speaking with.
For hidding muggleborns, or extremely saught-after witches, it was perfect.
It was useless, however, if the enemy saw her.
So, she stayed out of the field. Spent days revising her knowledge from The Books.
It was through this that she realized-
"Fuck."
-she'd screwed up.