For that Canon camera your wife has - my choice of a lens would be the Tamron 18-270mm f/3.5-6.3 Dill VC (anti-shake) lens - with this lens you not only have the ability of taking great landscape photographs - you also have the ability of getting close and personal with subject matter. This lens is not by any means cheap - but - it is a life long investment - this is a good walk around lens - making it a one lens carry. That being said - I carry a camera back pack with the following lens either on the camera or in the backpack - -Canon EF-S 55-250mm Lens IS \ EF-S 18-55mm IS and the Canon EF-S Macro f/2.8 60mm USM lens - if I had to choose one lens to put on the camera from those - it would be the Canon EF-S Macro f/2.8 60mm USM lens. Being a Macro lens - does not mean that you can only take Macro photography - it means that you "have the ability to photograph" those beautiful flower at a 1 to 1 ratio - bring home full life sized pictures - and it does a fantastic job on landscape photography and portrait photography. With the f/2.8 aperture you can blur out any background with a soft focus still leaving your subject matter in full detail or go to a large aperture (f/8+) to gain a full depth of field to bring everything in full focus. With today's computer programs (PhotoStitch) you can take panoramic pictures of anything. Hope your wife has a great trip - she is going to one of the most beautiful places in the world.
You didn't specifiy a budget but the Tamron 17-50mm f2.8 is an excellent lens with great optics. A newer 'VC' version was just released that includes image stabilization but costs more. The Tamron is quite an improvement over the XTi kit lens. Add a circular polarizer (Hoya makes decent ones) to make skies bluer and more vivid.