I am sorry to hear about your friend.
Hold old are her kids?
My daughter was not yet 3-1/2 when I was diagnosed with breast cancer last year. Books were the best way to communicate with her about what was going on. We read them over and over. There are actually quite a few picture books about moms with cancer. They are all fairly similar in plot (mom gets diagnosed, and then in varying order has surgery and is sore, has chemo and sick, tired and loses hair, and has radiation), but for us it was a good thing having a bunch of them. Some are specific to breast cancer.
Here are some I just pulled off her shelf:
Specifically mention cancer as being breast cancer:
Sammy's Mommy Has Cancer ((Kohlenberg)
When Eric's Mom Fought Cancer (Vigna)
Don't specify the cancer as in the breast:
When Mama Wore A Hat (Schick)
The Paper Chain (Blake, Blanchard, Parkinson)
Mom Has Cancer (Moore-Mallinos)
When Mommy Loses Her Hair It Means the Medicine is Working (Cervellini-Calfo)
When Someone You Love Has Cancer: A Guide to Help Kids Cope (Lewis)
The Hope Tree: Kids Talk About Breast Cancer (Numeroff and Harpham)
There is another book I couldn't find that I really liked, written by two little girls about their mother who was diagnosed with breast cancer, one of whom is named Abigail. Wish I could remember the title. The girls drew the pictures. I liked it quite a bit.
All these books have their charms. The picture/storybooks are all slightly different but enough the same that my daughter learned a lot through the repetitions of theme.
I particularly like "Sammy's Mommy" - the illustrations are very good. If I had to choose one picture book it might be the one.
The last two books are completely different and are aimed at slightly older kids, but they were still useful to us. I like them both in particular quite a lot. "The Hope Tree" is really more edited by the authors; it's a compilation of what kids in a support group, ages 5-12, have said. "When Someone You Love" has elves and chapters about different aspects of cancer, including dying.
One book I couldn't stand was "Tickles Tabitha." I was probably oversensitive to it at the time, but the father/husband is really rude to the mom who is tired from chemo. He doesn't ever acknowledge it in the book, and I just wanted to kick him.
"When Mommy Loses Her Hair" has an analogy I don't think my daughter followed, but an older child might. At the end it mentions faith and God which I didn't mind but the person who gave it to us did.
Also, in all the storybooks, the mom gets better. That bothered me a little. There is nothing I want more than to be done with cancer, but I can't say for sure that's the way my road will go.
I hope this is helpful.