Really recommend thermals, they used to laugh at me on camp, but I was always warm. If you can't stretch to merino, i have M&s in navy stretch in them so stay close but a little sweaty, aldi/lidl black cotton are cheap and have lasted well.
Layers are always recommended, thermal vest and some long sleeve cotton tops.
Go outdoors has been excellent for my unit, I even haggled discount when I met all 16 of my girls and tgeir parents there. 10% iirc. Waterproof shell trousers were good and dried quick, tuck into wellies too. Cotton combats are usually good there too.
Lots of nice thick warm socks, I often find I changed mine 4 times or so a day if it's wet/snowing.
Wellies are fine, as she get's more into Guides and camping then it's worth looking at sales and invest in a pair of £20 hikers, handy for walks and camping.
Check if hot water bottles are allowed, I use mine to warm my sleeping bag before I go to bed.
Carrier bags for her to put wet kit in, away from her dry goods.
Also rate highly aldi/lidl ski mittens & coats (salopettes as well) wear well and cosy fir the price.