Be sure to talk about her by name. As the days go on, others will go on with their lives but this poor family will still be mourning. Even though they know people haven't forgotten their daughter, it will seem like it when everyone avoids bringing her up in conversation. You will not upset them by bringing up her name. They are already upset. Not a minute goes by that they are not thinking of her.
Ask before you bring a certain dish. Especially if they are not pasta people, they can only eat so much pasta. It seems that pasta is the go-to dish people bring and it can get old. Ask if they would prefer enchiladas or a pot roast, for example. Ask if they have too many desserts as some people just bring sweets. Cut up veggies with dip might be nice as it can go with a meal or just have for a snack when they aren't really hungry.
See if they will let you help with the thank yous. Addressing, stamping, applying return addresses, etc. is helpful as they may have lots to send out.
Tell some of your favorite memories. Don't guess why the Lord took her. Just be sad with them and tell them you are so sorry for their pain and that you will be praying for them. Remind them that the Lord will be there for them during the difficult times and so will you.
Don't tell them to let you know what you can do to help. That is too hard. Give ideas and see if or when it would be helpful. Does your friend have someone to help her pick out clothes (if there will be an open casket)? Offer to come clean her bathrooms. Don't ask if she'd like them cleaned; just say you'd like to come clean them and ask what day would be best. She may be having lots of people coming over. Does she need sheets washed and guest beds made?
The more you can do, the sooner you will get over the shock. Then as you watch your friend, you will learn what her needs will be. Continue to talk about her daughter, long into the future. Keep reminding her she will get through if she says she can't. When she is not in a deep grief-stricken place, ask her what she imagines her daughter doing in heaven. Tell her what wonderful things you think she is doing.
I am so sorry for your loss and especially the family of the girl.