My mom had cancer this last yr. and just got over it a few months ago... well she told my bf something?

she told him not to tell me this but he coulodnt hide it from me ... mom told him that the dotors found cancer again and she wasnt doing the cemo again cuz shes allergic to it and to froget the radiation...my Q: is if you have cancer and go through it untreated will it spread for sure and eventually kill??? i want to kno cuz i have lost one person in my life every yr since 5th grade and i just lost my grandma i need to kno if i should prepair myself for the worst ....