I don’t know a lot about the libertarian framework and I didn’t read that specific book so maybe it’s not a legit question but: Who takes care of the vulnerable within a libertarian framework?
Discussion
Plead the cause of the fatherless and the widow.
Charity. Goodness comes from within.
It's not as if people are not already doing so. Iny personal capacity I set aside a monthly alotment of money, R1K, for charitable reasons. I then organise in my church what we do with it. And physically go give parcels of goods to the homeless and homed-but-less-fortunate.
Happy you asker BTW. It is definately an important question.