Can some pages be swapped with others based on viewport size?

Is there a way to present a different home page, or any specific page based on the size of the viewport?

I’m using the twentytwelve theme as a parent for my child site. Works great, responsive, etc. Problem is my client doesn’t understand responsive sites and is bothered that the images are small on her iphone… Duh.

So, I’m thinking, if she visits the home page (or other pages) on her iphone, she’ll see a page with a highly cropped version of the image (one designed specifically for the iphone screen). I’d need to present a different page (not just a different image) to do this, because the image is also acting as an image map.

Is there a plugin that would do this? Or perhaps some JS that would make the switch?