It think it is a good question and you seemed troubled by this and this person's choices in life. And that is the rub, they are choices. They know how to make them, they know the consequences, and they know how to work around them. Its a skill you probably don't have (and don't want to have).
In the end they will survive. If you go to a fast food store in a popular retirement location you will see plenty of elderly people working. They might live in low income housing, receive some financial assistance, and utilize other charities such a food banks. They might depend on family and friends. There is also the ugly, it is not a fairy tale that some supplement their diets with pet food.
There is of course social security. The amount is very low for most workers, but the amount is almost inconsequential. They would spend it all anyway and still be short despite the predictability of the income and a time frame with predictable expenses. Budgeting is a skill.
So I have a friend that deals with this himself, and is helping an elder relation. He and his wife provide some help, but when it started there was a endless stream of requests. His policy now is: No more help unless he works out a budget with the person requesting help.
I've used his ideas myself, and by using this it becomes clear on who is in actual need and who is just looking for the next handout. You can feel good about yourself for helping an actual needy person or guiltless say no.