My four-year-old son is currently undergoing chemotherapy for an aggressive lymphoma. His treatment has been very successful thus far. I cannot stress enough how important it has been for my family to receive the very kind support of so many in our community, includiing family and close friends, as well as new friends who have reached out to us since my son's diagnosis. Obviously, every experience will be different, based on the age and personalilty of the patient, as well as the family's desires. That being said, I hope your friends are using a caringbridge.org page, like others have recommended. We use it, and it is an excellent way to keep people informed of what is happening, as well as set the "tone" for the community to follow. In my son's case, the "cure" rate is very high, but is still a life-threatening illness. It was very important to my wife and I that people did not hear of our situation and think the worst - but rather, that they maintain a very positive outlook, as we strive to do. Whether you call it prayer, positive thinking, or something else, I am a big believer that the attitude, of the patient and others, matters. Perhaps the girl's attitude can be improved if the actual prognosis is not as bad as people tend to automatically think with "cancer?" Or with "beat the odds" stories?
To that end, you could offer to set up such a caring bridge page for them, if they have not done so or do not want that responsibility. You could later turn it over to them to update. You could also aid in gathering information about the type of leukemia she has, treatments, prognoses, facilities, alternatives, second opinions, etc. The first week was a whirlwind for us, and we felt it very important to gather as much information as possible about the illness, treatments, doctors, etc., to know that we were making the correct treatment decisions. I like to do that kind of research, so it worked out well for us, but some people don't like that sort of thing - maybe you could help with that? You could also offer to coordinate help from other friends. We were lucky that we knew several people who helped point us in the right direction for educationg ourselves and getting second opinions.
ASK the family how you can help. In our case, people volunteered to provide meals to us, so we would not have to worry about groceries, etc., while we were focused on our hospital stays. One person took it upon herself to start a list of people to provide meals, and then set up a calendar for which days they would provide them - she also asked our preferences on meal deliveries. Other family members and close friends offered to pick up the slack woth caring for our other child, since one of us is always at the hospital when our son is getting treatment. I don't know which type of leukemia your friend has, but there is likely to be lots of inpatient treatment - so there will be errands to run and other things that can be done to help the family. Some of my wife's friends chipped in for a housekeeper every couple of weeks.
Even if they have good health insurance, there will be significant expenses not paid by insurance, and perhaps less time for the parents to work if they work on commission or hourly, or need to take unpaid leave - so consider whether you can help them fundraise, relieve a financial burden (meals? groceries? take care of the dog?), or help them financially if they need it. There is a website called the "Human Tribe Project" which can help them raise some money via "tags" people can buy to show support for the patient: http://humantribeproject.com/
If they want to talk to someone going down a similar path right now, feel free to PM for contact info, and I will make myself available.