I do think the UX sketchbook is very neat - the squared paper inside the broswer frame is genius. It just seems perverse to only offer a single orientation in 2 different sizes. If it had portrait on one side and landscape on the other I'd buy it in a second (he hinted, in case the makers are reading).
So have I got a tip for you, friend. In case your wife hasn't found the right paper for her project yet, the best thing I can think of is to get a giant 11"x17" pad of Edward Tufte's grid paper. Scroll to the bottom of his shop page here:
http://www.edwardtufte.com/tufte/posters
Not only is it large and easily usable in landscape mode, it uses what he calls a "ghost grid", where the grid is printed with a barely noticeable light ink so that it's visually less dominant and it fades into the background compared to the ink drawings you make on top of it.
I used nothing but Tufte's paper for years and was deliriously happy with it. It was so big that I could use it for all kinds of stuff, whether site flows or detailed UIs, whatever. The only reason I switched away is that not every project needed that much real estate and it's not the easiest thing to carry around. It's like having to tote around a whiteboard.
If she wants something smaller and more portable, there's the dot grid book by Behance:
http://www.creativesoutfitter.com/product/34/dot-grid-book
Hope that helps...
If you really need to just design the first screenful of a page in landscape orientation, then go ahead and draw a big horizontal line through the centre of the page -- label it "FOLD" if it helps, and just work on that screen.