I used to repair these lenses when I worked as a camera tech at Canon, so got to use most of them at one time or another...

the best lens in terms of size vs quality is the little 35-70 f3.5/4.5, as it is sharp and offers very good macro across it's entire zoom range (and being plastic is very light)

the little 28-55 was also a pretty good performer, but not as good as the 35-70..

If weight is not an issue I would get a 35-105/3.5. I always found the 2 touch zoom (seperate focus and zoom rings) to be sharper than the one touch version, and if it is adjusted correctly then it will not focus shift through it's entire zoom range....

Or better still the FD 28-85 - these lenses are hard to find, but are a great general purpose lens.....

But considering she has the sharpest standard 50mm lens Canon made why would she want something less sharp? I'd recommend a NFD 35/2 or NFD 24/2.8 as her next wide angle lens, and a NFD 85/1.8 or NFD 100/2 as her next tele - these are all fantastic lenses