She knows pretty well everything there is to know on the subject.

pretty 在这里是副词，意思是"相当、颇"，表示程度较高。 well 是副词，意思是"好、充分、完全"，表示程度或状态。 pretty well 是一个固定搭配，意思是"几乎、差不多"，用来强调程度接近完全。