Check in your phone book for yarn supply stores (try Craft, Knitting, Wool, Yarn), and find one local to you. Then I'd have to say a gift voucher would be safest, or a pattern book, as you don't necessarily know what you friend is going to make, and It'd be a pain if she was that one ball short for the perfect project. and then there is taste in yarn: colours, fibres, Textures, weights, allergys-these are just a few of the things to consider. And i'm even worse when it comes to what needles I use.
Actully, take her on a mystery drive and hit the chosen LYS (or LYS's). Shopping with a friend to enable her addiction, would be the best gift of all.