Hi K Mitchell, welcome to Cruise-Chat. Sorry to hear of your wife's bad news. I went on my cruise during my chemo treatments. My oncologist worked around the time we were to be gone. Did you have cruise insurance? If so, your insurance should cover the problem. If not, then Princess did offer to reschedule your cruise. None of us plan to have cancer or anything else, but I always purchase insurance for the just in cases.
If you purchased insurance, check with your provider and see what they can do for you. We'll be praying for your wife's total recovery!