It seems at least possible that they made a pact: the only way that they leave their home is “boots first.”
some elderly people absolutely hate retirement homes and would never agree to live in one, no matter what. when i ran into this sentiment, i did not inquire why, but i suspect that it has something to do with dignity in death. before dying, they do not heed to worry about being embarrassed by not being able to take care of themselves and not being a drag to everyone around them. after dying, they of course do not care what happens to their mortal bodies.
so one way they could have gone is that gene could have made betsy swear not to call anyone for any help (once a wellness check is performed and it is determined by adult protective services that one or both elderly inhabitants are in medical danger, or the equivalent is passed along to authorities by a civilian). betsy honored gene’s wish. eventually, betsy succumbed to hantavirus (possibly due to cleaning such a large home by herself?). with betsy down and out, gene could not take care of himself, and conscious or not, lucid or not, he only lasted a few days without her. however, none of it was totally unforseen by either of them (with the possible exception of the dog that passed away in the kennel).
given the scenario i mentioned above, one could say that relatives should have intervened. however, elderly do not always give relatives a practical choice (at least until it is too late). to be effective, the relative or relatives need to be well enough off and within easy driving distance of the elderly in question. everyday life does not always allow such conditions.
also, santa fe is a tourist town and making a good living in the tourist industry for relatives is not always easy. people who choose to live in Santa Fe often choose it for privacy.
My very elderly parents moved into their house about 50 years ago. The original owner had medical issues and couldn't walk up the stairs to the second floor bedroom. An addition was built on the side of of the house as a new bedroom. The attic was was converted to a living space for a live in caretaker. There was also a buzzer system installed for requesting help from the caretaker. This was a 2 bedroom house until upgrades were made back in the day...
“possibly due to cleaning such a large home by herself?”
Being a primary care giver is physically draining. For a care giver for someone with diminished cognitive ability much more so.
We tend not to realize we can’t do everything we used to do when we were younger. It’s important for children to be aware and intervene when that happens. The care giver may not be able to make the leap.