Wherever you buy a Canon you may want to look carefully at the warranty. Often it is a local warranty only in the country of purchase.
I'm not sure about the "best" (I assume you mean cheapest) place to buy - I tend to be lazy when I need a new camera and just go to the most convenient place - last time that happened to be Wanchai Computer Centre.
Get her to look also at the Canon SX200i - it is a bit larger than the IXUS (which I like - IXUS is just too small and fiddly I think) and has an optical zoom that's the equivalent of a 336M lens on a 35mm camera. I'm very happy with mine. Should cost around $3500-3800 with the usual goodies thrown in (spare battery, case, screen protector, etc)