most jeans are sold by waist size, so it should be easy enough to get her a pair with 29 / 30in waist... be wary though, there are many many styles and shapes, so be sure to find out which she prefers (low-waisted, high-waisted, in-between...). Boot-leg suits most shapes - i.e medium-waisted with a slight flare at the end, to lengthen the leg.
Skirts are a little easier - a 29'' waist should probably equate to a size 12....
why don't you just take a look at some of her other clothes and check the size there?