I have one of the first-gen Canon f2.8 70-200mm lenses.
The quality of this lens cannot be overstated.
It is utterly sensational, even wide-open.

It is a large lens, though. If she doesn't truly need the f2.8 part, then the Canon f4 version is lighter and more portable.

If she does need f2.8, and can afford the Canon lens, then there's no need to even consider the Sigma. It may be a great lens too, dunno. But the Canon one is one of their very best.

Cheers