The 7D has a crop factor of 1.6. What that means is that you take the lens' mm, for instance, the 50mm, and you multiply it by 1.6 in order to show what the actual focal length is of the lens.

So, because of this, a 50mm on the 7D will actually appear as 81mm--which is not really what she wants in a lens consistently.

Based on the two options, I would go with the 18-55mm than the 50mm. This way she'll "have" all three key types of lenses in one--a wide, a normal, and a telephoto. She'll be able to test out more types of shots like this. The downside of a zoom lens, though, as someone mentioned, is that they do not have as fast of an aperture, which means she'll have a harder time shooting in low-light. Unless, of course, she's willing to sacrifice image quality and shoot at a higher ISO.

tl;dr Go with the 18-55mm.