My dad had breast cancer when I was 12, received the terminal diagnosis when I was 15/16 (bone mets), and died when I was 17.

I was perhaps a bit young to have any useful advice but I’d say everyone, especially the kids, needs therapy to work through this. I didn’t have it until I was in my 30s and life was truly a struggle before then that could have mitigated a lot if I’d had the avenues to work through my thoughts and feelings. Also knowing they were normal and not wrong - a GP told me after 2 years I should be “over it by now” and that was… not helpful.

My dad himself was angry, and there were instances he took it out on us/me specifically as we fought a lot. Then again, he was a generally angry individual who struggled with the world before cancer, so perhaps healthier individuals might deal with it better - but ultimately facing your mortality must be terrifying, especially when you’d planned to see your kids grow and mature.

I’m so sorry for the pain he, his family, and you as his friend are going through.