There is something called "Crop Factor". So a 35 mm lens will be the equal to a 50mm lens. A 100 mm lense will be equal to a 160mm lens. That is the only real difference and it has to do with the sensor. If she likes wide angle photography, then she may want to get the more expensive 5D for the full frame so that a 50mm is a 50mm. If she was using Minolta, then the lenses won't work digitally on the Minolta Digitals, as I understand it.
