I understand there might not be anything in the books to explain this, but I was wondering.
Slughorn repeatedly tells Harry of Lily's almost inspired abilities in Potions, while Snape, according to what he writes as the Half Blood Prince and the pure fact he went on to teach the subject pretty soon after leaving school, was clearly extremely advanced.
Considering the friendship lasted up to OWL exams, did Slughorn perhaps remember her being inspired up to that level and forget whether she perhaps fell back to relying on book smarts during NEWTs (which I think to become an Auror she must have taken)?
The description of Lily, while a great student, doesn't seem to suggest either the love, time or the pernickety dedication that Snape put into perfecting the subject, nor the unpredictability separating a student like Snape from a Hermione in terms of learning by experimentation over frequent journeys to the library.
So did Snape help her or was she just naturally talented at that as well? Or is it an impossible thing to answer using only the books as evidence? If so apologies.
We also know that Slytherin and Gryffindor students are taught together in Snape's dungeon, so it's possible, if not likely, both could have been in the same class.