Brain cancer, no. Breast cancer, yes...we found out my mom has it this past November.
Show your support for your friend through phone calls (just to talk, check in, say hello), cards, e-mails, any communication available to let her know you're thinking of her. Let her lead the conversation when you talk to her - if she wants to talk about her disease, let her be as open as she wishes. If she doesn't want to dwell on it, don't bring it up. Be a listener, a confidant, and a shoulder to lean on.
There are also charitable things you can do. You don't necessarily have to tell her if you choose to do them, unless you think she'd really appreciate it. Make a donation to the ACS or other cancer research institution. Join a Relay for Life team and walk for her. I'm walking in June for my mom, and I think it meant a lot to her when I told her I joined a team.
The treatments are hard on the body and the emotions. Expect it to be a roller coaster. Expect your friend to need every ounce of support she can get. There will be good days and bad days, but hang in there. Even from a distance, you can help her get through her battle.
Best of luck...and so sorry to hear this. That's such a young age to be given such devastating news.