A lot of it is trial and error, finding out personal preferences, but I think art supplies would be a great gift if she's only ever used office supplies! My family bought me nothing but art supplies several Christmases in a row when I was in high school, it was great!
For pencils, you're looking for graphite, a set of at least 5-6 would be best. (6B to 4H or 6H) They are labeled by hardness, which is what makes the color lighter or darker. A typical number 2 pencil is an HB, right in the middle. The price really ranges on these. Typically Prismacolor is one of the best brands for most supplies and they are all on sale at Michael's right now...I have their entire range of colored pencils and watercolor pencils and I love their markers, but I have a different brand of drawing pencils (actually a couple of brands), so I can't speak for them personally. For a basic set of pencils there won't be that big a price difference, especially if they are on sale, but most brands would be an upgrade from regular writing pencils.
At least as important, if not more, than the pencils is a good eraser. Or rather, erasers. These are only about a dollar total. She'll need a kneaded eraser (the gray squishy ones) and a gum eraser (white or brown hard squares). Some starting kits of pencils will come with these. I'd also get a smudging tool. These are just paper stumps for blending in sketches and will run you a dollar or two for a pack. Oh, and a good sharpener! Get a little all-metal one like the kind you used to use in school, just basically the blade and two pencil holes. Not plastic.
For paper, look for acid-free. Most art pads are labelled for what media they are best for as well as a weight. Printing paper is 50 pound paper, and for drawing I'd chose a 70 or 80 pound. (That just means that's how much a whole box of it weighs.) You can pick if you want to go simple or fancy, tear-out pages or not, spiral or book-bound, landscape or portrait orientation...totally up to you! They come in all sizes, as well. I'd probably go with a regular 9 x 11 or so pad (that fits in a backpack the best!), but personally I use my 18 x 12 and purse-sized pads just as much. Moleskin covers are usually the nicest looking, but you also have to think about what she's going to use. When I got my first really nice sketchbook, I was terrified to draw in it because I felt like I couldn't mess up! Some people like to do most of their sketches on cheap newsprint before they move to better paper, so maybe think about a pad of that as well. A 100 sheet 9x12 pad is like three dollars.
Remember that stores like Hobby Lobby and Michael's offer 40% off one regular-priced item coupons almost all the time, that you can print out online. Also remember that the people in these stores are there to help you! Most of them are working in a craft store because they craft. If the first person you ask isn't a sketcher, ask if they know another employee who is and can help you out. Most of the shoppers there will take pity on someone who looks really lost, as well, I've been on both sides of it.
Whew. That was long. Well, I hope that helped you!
Just PM me or respond to this thread if you have any other questions. I'm happy to help out.
