I would recommend definately going to decent-sized shop and trying various ones out. I used to work in one and we would fill up the bag with stuff so people could get a better idea of how it feels weighted, I think this is really important. There are lots of women specific or smaller size bags (normally shorter back, narrower waist belt and adjustable height chest strap) which are definately worth considering for her. However if you want to use it afterwards a couple of brands do adjustable bags which can cover a large size range.
80l is pretty big, if she doesn't need a tent/sleeping bag etc. (although a silk liner is always practical) 60l should be fine – it's often a case of if you have the extra space you'll fill it!
A filled 80l rucksack can often get up to ~30kg which is a lot if she's of a slighter build!
(if money is not an issue I can highly recommend macpac bags)