That she already has a pretty nice uke does make it a bit harder to find something that will really blow her mind. I still feel that you should go with a real instrument instead of a gift card. And I agree with those who suggested something complementary to what she already has. Although not as different as a vintage or a banjo uke. Those are very different animals altogether.
I'd stay away from Kala and Ohana, as these would only be a lateral move instead of a step up (except for the Kala Elite someone brought up). And I agree that Martins are not to everyone's taste. My suggestion would still lean towards a KoAloha Opio, because it is made of totally different wood, has a different look and the whole Hawaiian style to it. Or something like the
Romero Creations Tiny Tenor in all solid Koa wood? That one has a totally different look, is somewhat more portable, and many players swear by them. Other than that, I agree that the Cedar top Pono mentioned by others is a big step above the Kalas and Ohanas (I have owned several of each).
Again, if you buy it from The Ukulele Site, they will let you return it in case your friend won't like the one you got. This way, you can give her a real instrument, but have a backup plan to replace it with something she'd prefer over the one you picked. All your considerations and your willingness to raise your budget show that you greatly care about your friend. That alone would make me very, very happy as your friend.
Another thought: As mentioned above, I feel that a gift card is not the same as a real instrument. But how about inviting her on a date without telling her what it is, and then take a trip to one of the fine music stores - depending on where you are located, e.g. Uke Republic, Mim's, Elderly in Michigan, or U-Space L.A. in California -, where she can pick the uke she likes best (within a given budget)…? The trip alone would probably make for some pretty good memories.