You sound like a lovely friend, believe me your friend will appreciate the thought you are putting into this.
Favourite gifts we received when our daughter came home to us were things she could treasure and keep when she's older:
A name for her door, a wall plaque with her name (in your friends daughters case, yes it's really her own room and yes she is staying, it's permanent - something that isn't often possible in foster care)
A wooden memory box with her name on it.
Books were lovely, and as someone else mentioned, a bunch of flowers arrived for me, it meant the world to be treated as any other new mum. It's an emotional time for us too 
I really noticed the difference between my birth child (loads of flowers, cards and gifts) and my adopted daughter (age 9 months when she came to us), we received very few cards and gifts and I hurt for her because of that.
I know that your friends daughter will be older and therefore more aware, and it isn't wise for them to be bombarded by wrapped gifts and their transition to their forever family home needs to be low key and stress free, however you can still make gifts (given to your friend rather than directly to her child) meaningful so that she can treasure them later when she understands.
We received a few "aren't you great? We're so proud of you for doing such an amazing thing" type cards and they were NOT appropriate. The best cards were the ones that said "welcoming your daughter to your family" ones.
My birth daughter age 6 loves to go through her box of baby cards, it breaks my heart that my adopted daughter has so few. Her foster carer said that when she brought her home to foster family as a new born baby it was very sad to have no cards, balloons flowers for her and no celebration of her birth. So cards and keepsakes are very important.