It's clear that debtor's prison, en total, is unethical. It's also illegal in most of the Western world. And for cases where someone is imprisoned for any crime, the sentence should match the crime, and the treatment within prison should be humane.
My latter commentary is about the risk factors that this person knowingly took on doing business in that region. I personally, having traveled extensively, would never engage in business in the Middle East because I understand the nebulous way debt is treated in the region. Usury is illegal under Islamic law, and generally speaking debt is only acceptable in the context of direct personal relationships, which means any dispute about it is taken as a personal insult. Taking on a debt from the finance minister and member of the royal family of a theocratic dictatorship is not a good life decision.