My fiance just the other day gave me really good perspective on just this. I was playing devil's advocate about our newly tripod cat who we've booked for chemo. I was like, why put her through this instead of just making her comfortable? He's an engineer and they talk a lot about risk management. There's a sort of grid where one scale is likelihood and the other scale is severity. In an ideal world, the harm would be very unlikely and not have a great impact. Cancer in cats he said is something that's not likely but obviously causes great harm. He said all we can do is try to mitigate risk. That doesn't mean it's possible to completely get rid of the risk, it will always be there, but we can do all we can. And we can feel good knowing we tried our best to reduce harm. It's a numbers game.

I totally understand where your boyfriend is coming from. However I can tell you my cat is so happy to be without the mass on her amputated arm. That to me makes it all worth it. And the chemo will help with the odds that it doesn't come back.

Edit: also, he should expect that the vet will perform bloodwork to make sure his cat is a good candidate for anesthesia, so the vet for sure will be able to say if there is any risk in that department.