If he wasn’t treated with hydroxychloroquine before death, lawsuit city.
They would have zero chance of winning.
Although a doctor can prescribe a drug off label, he could not be held responsible for a bad outcome due to not using a drug off label.
If a drug is not approved for use against a given pathogen a doctor is not under any requirement to consider its use in a case where that pathogen is present. Most doctors will not prescribe a drug off label (other than anti-biotics which far to many do).