Here is my first attempt at answering some of your art questions! More to come soon. Thanks for the comments and questions; please keep them coming! :)
I guess it would depend on what mediums your friend worked with, but if they do any kind of painting, an artist can never have too many paintbrushes, especially ones for detailed work. Don't get the cheapest paintbrushes at the store, and stay far away from the most expensive ones, lol... I would say $3-5 per brush would be your best bet - not too cheap and not too pricey.
(Above is a photo of a painting in progress with some of my favorite brushes. Ooh, I need to work on that ear!)
Resource books are always great too... or gift cards to online art supply stores for sure! What other ideas does everyone have?