I was diagnosed with breast cancer last December and finished chemo a few months ago. Having just been through this, I can tell you that going through it with young kids (mine were 18 months and 3 years old at diagnosis) in the house is hard. Some of the best things people did for me was to bring food and help get my kids to/from school and daycare. I couldn't drive for almost 2 months after my surgery (had a double mastectomy) and couldn't hold/pick up my kids for that long either, so I wasn't able to take care of my youngest myself and I am a SAHM. We eventually put him in full time daycare.
The best things you can do are listen when she needs/wants to talk, bring food and clean her house and help get her girls where they need to go, especially if they are involved in extracurricular activities.
Have her set up a CaringBridge or other similar site so that she can post updates there and not have to tell the same story to every person that calls. This helped me immensly so that people knew how to pray for us and help us without me having to talk about it nonstop. Also, there is a site called CareCalendar.org and she can setup a calendar of when she might need food, childcare, rides to doctors, etc. and people can get onto the site and sign up for the things she posts. Maybe you could help by being her coordinator for this site so that people can call you with questions instead of her. Helping her organize all of this would be a huge blessing.
If you have any more specific questions (or if your friend does), feel free to send me a PM and I am happy to give you my personal contact information. Sometimes it helps to have someone to talk to that has been there, so I'm happy to talk to your friend if she would like. Hope this helps!