Alcohol as necessary medicine when visiting dry countries?

Whether any country will recognize the user of alcohol for medicinal purposes presumably depends on that country's medical regulators. Dry countries might allow the user of alcohol on a foreign doctor's prescription, or they might require the prescription of a doctor licensed to practice medicine in that country, or the regulator might only approve alternative therapies (for example, to interfere with the enzyme in question) that do not involve alcohol.

It will depend on the country. In Brunei for instance, non-Muslims can import twelve cans of beer and 2 bottles of liquor, every 48 hours (when doing a Labuan run for instance). You can only consume the alcohol inside your residence, hotel, etc... You won't be able to buy any in-country.

Other countries can be stricter, and it would prove dangerous to break the law. But in all muslim countries, doctor's orders won't fly, especially a foreign doctor. A person needing alcohol for medical reasons would have as much leverage as a person with a cannabis prescription in Singapore. Zilch.

If the person in question truly needs alcohol as medicine, it would be best to purchase it in a medical container rather than carrying a bottle of whiskey in your luggage. It is best to ask the doctor in your country about how to obtain medicinal alcohol, but I expect it to be packaged similar to ethanol used for scientific research:

Of course, you'd still have to declare it as a medicine at each border, but I expect the border guards to be more lenient if it appears as something that could actually be used for treatment.