His treatment is unacceptable, the sentence is outrageous (and counterproductive), but fundamentally this is why you don't get into business with and especially don't defraud petty dictators in unstable regions of the world. If this had been in his place of birth, he would have been murdered rather than imprisoned, and I imagine he should be well aware of this given his upbringing and career history.
"he would have been murdered rather than imprisoned" is also not especially comforting considering just how badly one can be scarred by imprisonment, interrogation, etc.
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.