I think she needs to hear it from the vet. *If it's really time, the vet
should make it quite clear to both of you together. Make sure he's
specific and not vague, so there's no doubt in her mind. *Sometimes we
hear only what we want to hear, especially at such a difficult time. *
I'm sorry you have to go through this.

I agree with Bobbie and LauraM. Ask the vet for an honest opinion.
Sometimes, the owners are too close to the situation to make a
decision, and this is probably the case with your wife.

I'm sorry you're in this situation. Best wishes to you both.