Best friend diagnosed with stage 4 glioblastoma and dealing with it

So this is the first time in over 15 months me seeking a support page or finding people who are going through similar things. I think it's because I'm only just now realising, I maybe need some help in dealing with this. First, I will tell my story and all my feelings surrounding it. 

I am 28 and so is my best friend I will call G. We have been the best of friends since we were 4 years old. Me and him have had other best friends in our lives but he is the ONLY friend I have ever had that I could tell anything to. All the small little thoughts in my head whether good or terrible I would say it to. I knew him inside out and him the same to me. He would meet me daily when I finish my shift at 11pm we would chat, and he would walk me home.  

For about 7 months I knew something wasn't right with him. Small things like a tremor and depression (I pushed him to go the GP). After months of pushing, he went, and they diagnosed him with vitamin D deficiency and maybe depression. I didn't take that for an answer and pushed him to go back. After it got really bad, and I KNEW in myself something was really wrong. I even sat down with my family and cousin (who is also his close friend) and said something was neurologically wrong with him. I even said a brain tumour. Everyone told me I was being silly and dramatic. Then one day my cousin sat down with me and said I think you're right there is something neurologically wrong. I said, so what should we do, she said let's wait for him to go work this weekend and we can sit with his mum and tell her we think its urgent. That SAME day he went to work, and he couldn't walk up the stairs. His mum took him to A and E where he had a mass on the brain. After the biopsy a month later, it was found to be cancerous. Glioblastoma. Inoperable. I don't even want to go into that period of time because it’s too painful. He done 6 weeks of radio where he was really sick. The dynamics of our friendship had changed. I was now walking him home and taking care of him. 

That time you see who your real friends are. In the first 2 weeks after his cancer diagnosis, he had all “friends” all around him. But when it gets nitty and gritty, no one was there. It’s hard to hear your best friend has cancer and probably will die in a year best, and to just go back to work and normal life. It's really affected me. But every day at 4pm on my break I will call him. When he's going through radio sitting with him whilst he's puking in a restaurant and joking about it to try to keep the same humour our relationship has always been built on. When the cancerous brain tumour has made him aggressive and rude... I’m still there. 

He was given 15 months with treatment. After the radio he done chemo. He finished all the chemo that they said that they would do in this June just gone. Had a scan and they said a tiny decrease, but they won't do any more treatment. He has another scan in September where they are just keeping their eye on it. To me he is good and not really sick, so I don't believe the 15-18 month prognosis they gave him. I think I’m just venting but if anything, I want to ask is how long do you think he will live if he is still good in health and spirit? 

For me it’s really difficult. I feel like I'm going through grief already. Not of him dying and not being here, but grieving the version of him he was before, that I have lost. But when eventually his time comes, I am going to go through it all again. 

I don't really have any questions I guess I am just venting, and I will maybe seek help for my own mental wellbeing and how it has affected me. I feel ready for that now. I guess the only question I have is how long do you think a 28 year old male who was diagnosed with cancer (stage 4 glioblastoma) in January and started treatment in June 2023 and who is still in good spirits and good health as it seems will live? I know you can't really answer me but anyone who has gone through the same thing. Am I looking at months or years?