Yes -- but it doesn't work the other way around.
Canon EOS "EF" series lenses (including all your "L" series lenses) will work on ANY Canon EOS camera body regardless of whether it's film or digital and regardless of whether a digital camera has a full-frame size sensor (like the 5D which has a sensor the same size as a 35mm film negative frame) or a crop-frame camera (like the Rebel series bodies that use the "APS-C" size sensor which is a little smaller.)
But suppose you get your GF a Rebel body... it will likely come with one "kit" lens (usually that kit lens is the EF-S 18-55mm f/3.5-5.6 IS II or EF-S 18-55mm f/3.5-5.6 IS STM). But that lens is an EF-S lens. The image that the lens projects into the camera body is large enough to fill the sensor surface from corner to corner for an APS-C sensor... but it's NOT a large enough image to cover the size of a full-frame sensor from corner to corner. EF-S lenses' advantage is that they can produce a high quality image that doesn't need to as large as an EF lens, and because of that, they're able to reduce the manufacturing cost of the lens and THAT means the lens can be offered at a lower price point than the EF lenses of roughly similar focal length ranges.
Those EF-S lenses can be used on any Canon EOS digital camera that has an "APS-C" size sensor (any Rebel body, also any of the 20D through 70D series, as well as the 7D bodies) but cannot be used on any camera body that uses a larger sensor (any 5D series, 6D, or 1D series body requires "EF" lenses.)
The specialty lenses such as the tilt-shift (TS-E) and the extreme macro-photo lens (MP-E) can be used on any EOS body (just like the EF lenses.)
Tim Campbell
5D III, 5D IV, 60Da
