I would agree that if this is the only time they ever discussed the matter, then yes, this should not be legal grounds to go forward with the removal of the feeding tube. I just find it hard to believe that after many many years of marriage end of life issues were never discussed beyond this.
What do you consider many years? They were married for five years when Terri suffered cardiac arrest. To me, that isn't that long and they were very young at the time. My ex-husband and I never discussed those issues when we were in our early 20's. It didn't cross either of our minds that we might end up in a situation like that. My second husband and I have discussed it in detail because I'm 38 and he's 41 and we are much more aware of our mortality than we were when we were in our 20's.
I am 46 years old and have been married for 27 years. I had never discussed this subject with my wife until the last month or so because of all the publicity surrounding the Shiavo case. Since neither of us had ever had a relative or friend in this situation, the subject just never came up.