My situation is similar. My wife was mostly functional 2 weeks ago. Last Friday I took her to the ER for extreme fatigue that developed over the course of a few days. I thought they'd do a transfusion or 2 and let her go home. Instead they hospitalized her and did CTs, x-rays, etc. Those results showed significant cancer advancement, especially in her liver. At this point treatment would shorten her life, not lengthen it.
2 months ago my wife was 'normal'. She has brain mets, so for safety sake we restricted her driving. 2 weeks ago she was eating and cooking. Today she's stumbling, falling, slurring her words at times, not eating solid food and wearing Depends. Yes, it can change quickly. The only way I can figure it, is once the cancer gets the edge, it seems to take over.
I definitely agree with Matt. you need more than a nurse's opinion. maybe she was in the discussions and is right. maybe not and it's just a guess. either way, you need facts for their basis. if we didn't have the scans and x-rays, I wouldn't believe it either.
I'm know this post probably isn't encouraging, but please know that you aren't alone. It sounds as though we may be walking the same path together. Hang in there.