Oh my goodness how scary for them. I'm glad to hear it's likely treatable if it is malignant, but still not the news you want to hear.

I agree that you don't necessarily need to bring it up at all. Just say that you're there for her if she needs you and suggest specific things that you can provide- food, a trip to her house to bring clothes to the hospital, take daughter out for an afternoon, etc. I've been volunteering in a children's hospital for the last couple years and I've found that, especially with parents that just got there, it's a lot easier to get them to accept help if you're specific. I can ask "do you need anything?" and 95% of the time the answer will be "no." But if I ask "Do you need toys for the kids? A break? Some toiletries? Any clothes?" suddenly the chances of them saying yes to one of those things is much higher. Parents who have been there a long time are much more likely to ask for things up front, but parents new to the gig are trying to be strong and do it on their own. But chances are they grabbed the kid and showed up at the hospital and they may not be wearing shoes, much less real clothes or have packed a toothbrush. So that's what friends, family, and your hospital volunteers are for.