Out of curiosity, what's the perceived problem with the push-pull design? If she's tried it and hated it, fine. But it has the advantage of allowing a very fast change in focal length. Also, it's a 400mm lens with IS that stores more compactly than the 300/4 IS, and much more compactly than the 400/5.6. I owned the 300/4 IS for a while, switched to the 100-400 for the extra 100mm, to keep IS, and for the versatility of the zoom. So, I'd recommend the 100-400mm over the other two.
Else, I'd pick the 300/4 IS over the 400/5.6. The 400mm lens is good for birds in flight, where you need a very fast shutter speed so the lack of IS isn't a huge issue. The 300/4 IS also has a much closer MFD, so it's good for close ups of flowers and butterflies (as is the 100-400, but not the 400/5.6). If you like, I can post samples of the 7D + 100-400...that lens pretty much lives on my 7D now.