I was in San Andres last December - it's such a wonderful place! My Boyfriend actually asked me to marry him while we were there Perfect setting.
I found things to be really cheap there (coffee, rum and vanilla extract). I took money out from bank machines there and had little trouble (you just have to find the correct machine, once you do - it's just like taking money out at home!) I probably took about $50 us with me and took out another $100 there (I think I still have some left over!) Then again it all depends on what you want to buy.
Tip: when shopping with Credit cards some places will charge you extra for using a card.
Everything in US bills. Don't tip in Canadian money, it's very hard for them to change it there, we changed some since we'll end up using it anyways, but some staff actually looked for us to change their Canadian money.
Tell your daughter to have fun! I'm sure she will!