Shame on them. Hope they got the Germany Flag right when they went to the Embassy.
I know Tricare covers covid you could look it up on their website. They will not give you a letter and the American Embassy has done nothing to help us. I am also covered by Tricare for Life. You need to get insurance from one of the Thai companies they have listed to cover you for an O/A visa or get a new visa type O for retirement or marriage to get out of having the insurance. Being 80 your father will have a hard time getting insurance but if he must get the cheapest with the max discounts.