I'm so sorry to read about this. My son had a brain tumour when he was younger and is now doing well. FB has some very good groups of parents with kids with different tumours. In the early days it can be a bit confronting to read the posts, but those kind of things are a source of great support and information for many people.
If you don't mind sharing, what were his symptoms please? I have concerns about one of my children. The GP was dismissive and implied I was an over anxious Mum. I know symptoms can vary but id be interested all the same.
and the second is the chapter 'Psalms' in the Bible.
An actual physical bible where your friend can browse the psalms would be best. Most new testament bibles have psalms (indexed) for times of fear/sorrow/suffering/anger/guilt/injustice. I find psalms soothing despite not really believing in God.
It's something to do. Even if you just argue with the bible. I just found it something to do.
I think just be there, as it sounds like you are doing. Help with the practical stuff as much as you can: clean the house, or babysit siblings, or freeze some meals for them. Pass on news to others if they don't want to do it themselves. Maybe organise some others to help too if you can. I am glad they have a good friend.