Condolences. This is a tough situation all around. In answer to your question, I have first and experience, I was diagnosed with Stage 4 Colon Cancer around 2013 and posted a bit about my experience.
https://www.fohcigars.com/forum/search/?&q=Cancer&author=Elvis&search_and_or=or
In my case, the cancer could have been detected much earlier if I just would have had an annual physical/checkup with my doctor. My wife kept making appointments for me and I kept blowing them off because somehow I thought my work was "more important". So please everyone, get checked out yearly.
As far as cigars go, I can't imagine they would mix with chemo. Everything tasted like tin and I lost all appetite. It was altogether an unpleasant experience that I would not look forward to repeating. In general, I don't think it's a good idea to smoke when convalescing and your body is trying to heal itself, but that's just my opinion. I can't imagine any U.S. doctor going on record as saying that any amount nicotine consumption is acceptable, those are just the times that we live in.
Choosing what risks to accept are very personal decisions that define us. I made mine but they are certainly not for everyone. I fully expect that cancer will resurface in some form or another whether I continue to smoke or not. Every five years are so there seems to be generational advances in the treatment of cancers which gives me reason to be optimistic.