Let your friend know that you will be there for her both now and later. And then, well, BE there. Offer to take care of her other child, be there while she is with her son in the hospital, cook, clean - better yet: find other people to do these less personal domestic things so you can physically and emotionally be there for her.
People say "brain tumor" like it is the worst possible thing that can happen to a person, but that is not the case. When I was 27 I was diadnosed with a brain tumor - 15 years ago. It was removed and I was out of the hospital in 3 days. Elizabeth Taylor had a brain tumor and is still going strong. There was really no physical pain for me - the head does not have a lot of nerves. And for me, personally, after being tested and put off by doctors for almost a year, the diagnosis was a relief. I wasn't going insane, and I did not have cancer. The hardest part for me and my family was the fear: "brain tumor" sounds very scary. Probably because it is not as well known as say breast cancer.
Find out information about what is going on and what the outcomes might be - she may not be in a state to handle this stuff. Be positive and comforting without shoving happiness down her throat. Tell her it is alright to be afraid.
There's lots of good news medical wise: big advancements in technology - many times they do not even need to make a single incision. Children's brains have a great capacity for regeneration if there is any damage. And it is a blessing she has a good enough doctor who took the extra step to find out what was really ailing her son! It is wonderful that the tumor will be taken care of so quickly - less time to worry.
You could make her a soothing tape, CD, ipod download. She may have a lot of time on her hands at the hospital. She may need time alone, she may need a shoulder to cry on. You are her good friend. Follow your instincts and give her what you think she needs but respect her reaction. And take care of yourself and be brave. The hardest thing sometimes is to turn toward a person who is going through a medical crisis rather than away. A lot of people stay away and say, "I didn't know what to say," when what a person needs is simply to have you turn toward them with open arms and not say a thing.