I would go through and sort out all the uncommons (diamonds) and rares (stars). Out of 1200 there are likely only 300 or less not counting duplicates. Then start looking them up (make her do it on tcgplayer, if she wants to make the money), and keep an excel sheet. The rares are usually the only ones that command any real price. However, there are some uncommons, like the Propogation Exeggcute that will sell for $7 or so. Once you have a list and an idea on price, then find an event near you with the pokemon event finder and contact the organizer or show up and ask the best way to get the word out that she wants to sell her cards, provide a list (or pictures in binders), and a price for the whole set or ask for offers.
I've seen sets sold all at once and in pieces. I think the better way is to sell the whole set if you can find a buyer. If you sell them piecemeal you are going to be stuck with the cards only hardcore collectors want. They might be willing to buy them for something (better than nothing on a sunk cost) because they may have an easier time getting value out of them later.
If I were to sell all my cards, I would keep track of all the cards that were selling for $3 or more on tcgplayer and set that as my initial lowest sell price for the set, and negotiate from that point.