Most of what I'd tell you, she already has.

Not all thrift stores are equal. Here in Davis, there are two stores that put all others to shame: the AR&R Thrift Shop and the SPCA Thrift Store. Visit all the stores in your area. Check their prices, their selection, and the quality of the clothes there. Wealthy cities tend to have nicer clothes in their shops and, oddly enough, better prices too.

A good store, by definition, has a fitting room.

While you're at it, find out if there are special times to shop. The SPCA store in Davis, for example, puts much of its inventory on sale each Wednesday. Other stores may have a specific day when unsold merchandise is marked down.

Be prepared to spend an hour and come home with nothing, or with an armload of things, as the luck of the day's selection dictates.

Next, you have to learn to overcome any clothes-finding challenges your anatomy may have handed you. You'll need to learn your own set of tricks, but maybe mine can help you -

- Bring a string cut to the length of any hard-to-fit measurements. I have longer arms than most women, so it's hard to find tops with sleeves that fit. I use a string cut to my ideal sleeve length; by holding it up to the sleeve of potential buys, I save heartbreaking trips to the changing room.
- Wear nylons or knee-highs so that you can try shoes easily.

Charity thrift stores usually rely partially or completely on volunteers. I've worked some at the AR&R Thrift Shop, and I highly reccommend it. Consider the benefits -

Inside information - seeing the workings of a thrift shop from the inside gives you a better feel for the way they work, which will help you in your own shopping.
Fun people - after all, they're thrifters like you!
Save the world - it's a contribution to whatever cause the store supports.
Shop while you work!