Thats so sad. Every parent's nightmare. I've never experenced it, thank God, but I have know people, not to directly (Mother's friends), who have experenced it.
The only thing anyone can do for her is be there to try and comfort her, it's going to be really rough for a long time. She also needs to be reasures, possibilly repeativly, that it's not her fault, and that there was nothing she could to. I think she also needs hope. Maybe knowing that oneday it wont hurt as much, that oneday things will be eaiser will help her. There isn't too much you can say or do, It's something she will never forget, or getover. I think she just will really need all the comfort and support she can get. Someone to be there when all she can do is cry, someone to talk to, and someone to just be there for her.