Partially correct. People do miss departures, and this is a tourist island with hotels with restaurants where a passenger could overstay a meal or shopping.
However it is the tour providers responsibility. They do keep track of the number of tourists. If one was missing they should have known when boarding the return transportation and mentioned they were missing a passenger. Not that unusual, but had the cruise line known that, particularly when discovering her age, at the very least the line could have notified their port agent and/or local authorities. From the little information provided this might not have saved her life. And she should have told the guide she was stopping, no way of knowing if they did, in which case they likely would have gone back to find her. All kinds of disclaimers in the cruise contract and the tour disclosures, but the cruise line has the deep pockets in this instance.
Should have noted other articles indicate she tired and headed back on the trail herself. Whether she told anyone, we’ll never know.