I agree that Prismacolors are the absolute best. A million colors too. They also have a range of markers and pastels. I know it's a little dry but you can also get her a gift card if you can't come up with specific ideas or even take her on an art shopping trip. That's my personal favorite. Then she gets exactly what she wants and you get to spend time with your friend!
If you ask me 'what are the best tools' then I do not know. But these are just personal preferences :> - Wooden pencil ---> quick sketch, caricature, storyboard - A mechanical pencil ---> comic/manga * (HB-2B)
Depends on what medium she likes and hard to know what she likes, unless you give us a bit more details, since there is alot of traditional mediums she can do.
The best guess though for most artist is probably they sketch alot, so that can help. Any supplies for sketching would be greatful. But introducing a new medium should be a joy for alot of artists too.
Maybe you even get her a white and black pen, and a brown tone sketchbook. I know alot of people in industry like to work this way since it brings alot of hightlights and the ink out more. Or get supplies for gouche with bristol or pastel paper. Or canvas for oil or acrylic. There is endless possible things you can get her.
Pretty much anything strolling down from the artist Isle, would be joyful day for an artist.
On pencils for drawing/sketching, I highly recommend Staedtler Mars Lumograph. They have from 2H - 8B, and are amazingly soft. Koh-I-Noor is always mentioned but I think it's too dry. On color pencils, I have several recommendations. If water-soluble, don't think much: Staedtler Noris Club (if you're looking for something cheaper), Faber-Castell Albrecht Dürer or Caran D'Ache Supracolor Soft II. If normal, I am very fond of Faber-Castell Classic Colors and Uni Mitsubishi colorpencils.
You should probably ask your friend! different people love a wide variety of supplies. I personally love simple mechanical pencils with refillable leads and a matching mechanical eraser. Some folks love colored pencils or brand name standard style pencils that require sharpener.