Define soon. It's not like she is going to drop dead tomorrow or in a couple of months. It is possible that she could be alive and well 10 years from now. The odds may not be favorable, but people beat the odds. So how, exactly do you think it would help her for him to say "hon, I'm staying home because you are going to be dead soon?"
Denial (from Kuebler Ross) is perhaps the most difficult step in accepting your mortality, then anger. She will resent his not being there for this process. A freind was diagnosed with lung cancer, six weeks later he's gone. He accepted it quickly and was able to pull together finances and other matters so his wife wasn't overwhelmed when the end came.
So yeah it would help for him to be there because she may be dead soon.
It's not so much he needs to be with her as she dies, but to be with her as she lives, and with luck survives.
At least that's what I would do.