This is a very hard conversation to have, when the doctor acknowledges that cancer treatment is no longer achieving results. You and your father have to weigh the negative aspects
of treatment, such as uncomfortable and painful side effects, against your father's need to keep seeking a cure. It's one of the moments in care giving when the role reversal is very intense -- you're going to have to be the parent in this situation, and help your father decide what's the best thing to do, just as he did with you when you were young.
The fact that your father's oncologist raised the subject is a sign that she's a good doctor. Because oncologists are in the business of curing cancer, many times they feel such a sense of failure when a patient is getting sicker that they just keep treating even when it no longer makes sense.
You could start by suggesting to your father that the two of you talk to the oncologist again and ask a few specific questions, such as, "Is continuing treatment going to make a significant difference in the progress of the cancer?" You can also ask, "If we discontinue treatment, can we expect the nausea, pain, and other discomforts to diminish?" If the doctor answers that treatment at this point won't make a big difference, but discontinuing it will improve your father's quality of life, that may help clarify the decision for both of you.
You may find that your father is more ready to talk about these big issues than you think. So often for family members it's actually their fear of bringing up difficult issues that's getting in the way, not the cancer patient's refusal to talk about them. We as a culture are death-phobic, and family members are very fearful that stopping treatment will be perceived as "giving up" on the patient, when it can be just the opposite.
Sometimes this moment can be the catalyst for a really important conversation; you could ask your father, "If you only had six months or a year to live, what would be most important to you? What can we do to make it as wonderful as possible?" I always remind people that there's a difference between hope and what we call "expectations for outcome." Just because the treatment is no longer expected to bring about a cure doesn't mean it's time to give up hope. And as time goes on, hope can be about being out of pain and suffering.