User blog comment:Miskos3/Fairy Tail Chapter 486: The Fourth Guest, Review/@comment-4507125-20160524002409/@comment-7418318-20160524074121

Natsu turned down the help of the Twin Dragons against Mard, who were allies and friends. And that "mission" was to take a book for his father.

Now we are talking about potentially accepting help from an enemy in an attempt to get revenge on the one who killed his father. That's a few levels more personal than retrieving the book so I seriously doubt Natsu would be willing to accept her help... even more so if he's already in rage-mode plus Dragon Force.