Mum recently diagnosed

If your mum is still smoking it is quite possible she is using it as a stress relief against depression. The breast cancer alone is enough to bring on depression. Ask the oncologist if they have a resident psychologist available. It may help your mum to understand why she is smoking and put in place some different mechanisms to cope with stress.
I did ask for one but it was after I had finished chemo and my GP said I no longer qualified to see the cancer psychologist. I gave that GP the flick and got a new GP who I only discovered yesterday is a psychologist as well. I might finally get the help I need and stop smoking too.