I am so sorry for your friend. I can't think of much worse loss than that.
Writing a letter and relating how her daughter impacted you (if you knew her) would be great. If you didn't know her daughter too well then a letter telling her you are there for her and will be checking in on her regularly will be great. Food is great, especially if she has people at home to cook for. If it's just her or her and her husband, she may not want or need much in the way of food.
I experienced a similar situation. A dear friend lost her fight with a rare and aggressive form of breast cancer. Her mom and I have become better friends through this loss. I try to remember and send cards on the anniversary of her daughter's death as well as her birthday. I tell her funny stories of things her daughter and I did together and how she made me laugh. I listen when she's sad about some aspect of having to return to life as normal even though her life, as far as she's concerned, will never be "normal" again. She's a tough lady and she's an inspiration to me. I remind her how much her daughter loved her.
So, I agree with others. She'll need your support more down the road a bit. Right now she's in shock and probably has a lot of people around her for a period of time. Later, when everyone gets back to their routine, there will be another wave of grief even more than right now.
You are a great friend and you will know the right things to do as they come. Be available to listen and offer a shoulder. Maybe invite her to your house in a few weeks for a change in scenery or meet somewhere between your two places for a weekend or something.
Whey my husband's mom died, my mom wanted to plant a tree in our yard in memory of her. It was a beautiful sentiment and meant so much to my husband. His mom was a master gardner and there could be no better memorial in her honor. My brother in law has a memorial garden at his house and plants something new for every person who has passed from his family. My oldest brother in law had a widow maker heart attack in May last year at 59. We were devastated to say the least. When I visited Jim's house, he had planted a gardenia bush in John's honor. They were John's favorite. He has a magnolia tree for his mom and another flowering bush in honor of his aunt. It is so touching to me. So maybe that's an option.
Blessings to you as you walk this road and take care of your friend.
L.