These conversations are always so difficult and hard to start. I agree with all the contributors the best way is to comfort your mother and let her know you are here for her during this time of need. Communicating to her that you are her support system and will support her in her decisions of care is an important way of letting her now your there, and giving her control of decisions when she probably feels there is little she can control. As Carm said the doctor wants to stop chemo that doesn't mean other options cannot be investigated or other treatments if that is the direction your mom would like to go. I am a strong believer in second and third and as many opinions or conversations you need to have with a doctor until you are satisfied with the amount of information you have to make an assessment of what your would like to do. Sometimes also hearing information from different resources and perspectives canhelp clear up misconceptions simply by the presentation of the person giving the information.
You are in my thoughts and prayers!!!