Most have been mentioned but here's my list (I hunt in northern wisconsin so when I think cold I'm thinking 0-20 degrees or so)

1)Long underwear that wicks away sweat - oddly enough I think silk is a good material for this? I think you're on the right track with the under armour though.

2)Socks - wool, wool, wool. NO COTTON... I use a nylon or silk sock as my first layer, then a lighter wool, then a heavy wool. I'll even use toe warmers withthree pair of socks - no problems with toesgetting cold then. Another piece of advice is to have boots that are big enough to fit all these socks in. Tight fitting boots will make your feet get colder faster. I usually buy a pair that is 2 sizes bigger than my "normal" size.

3)Wool sweater.. best investment I've ever made. Woolrich makes a nice lightweight wool sweater that does great job with not a lot of bulk

4)Bibs of some sort.Heat loss from where the jacket and the pants meet can be an issue.. bibs take care of that.

5)A warm hat or facemask. Keeping your head warm is a must, keeps the heat in. I also like to keep my neck covered with a turtleneck of some sort so the wind can't get on it.

6)Hands - I wear a wool mitten on my bow hand and a lightweight glove on my release hand. I use a hand muffler with chemical handwarmers and usually keep my release hand in there.
Using a bow hanger helps tremendously here also. The bow gets so cold and can make you get cold fast when you hold it. The chemical warmers help tremendously in that case - I'll stick them in my hands to warm them up after holding the bow for a while.

I don't think you need to spend a lot of money on these things.The wool is the biggest factor in my mind.. and the assistance of the chemical warmers, they are a godsend. Stay away from cotton of any sort - socks, sweatshirts, etc. The basics are feet, head, and hands.. keep those warm and you'll have a good start.

ah yes - the most important thing - make sure she gets to see deer! That will make you forget about being cold for quite a while!