When me and my bf celebrated our 6-month anniversary, I surprised him with a candle lit dinner cooked completely by me! He didn't buy me a gift or anything, he simply wrote me a really sweet poem, which meant more to me than any gift he could go out and buy.
I don't know this girl, but personally, I prefer simple, but thoughtful gifts that show that he cares about me. Try writing a poem, burning a CD of both of your favorite songs, or stuff like that. Something that shows how well you know her, or how much you care about her, etc...
Hope I helped!