What does she shoot? At which stage is she in photography?
For the average hobbyist, upgrading from a rebel to a 5Dmk3 seems idiotic to me. It costs half a child, and unless she needs very good auto focus and extreme high ISO capabilities, on a full frame sensor, the mk2 would probably be more than enough. Frankly, I'd even consider the mk1. Still full frame, good image quality, though a little limited ISO range. I've been considering selling my 40D and getting the 50mk1 and some used L glass, myself.
Again, it all depends on her needs.
"I can resist anything, except temptations."