Never heard of a drug that lasted two weeks, and I have worked for vets and still do for over 30 years.
Unless a drug is given daily or even every other day, there is not a drug that will stay in a horses system for two weeks. There are not human drugs that stay in a body for two weeks. Doesn't make sense at all. If the horse was indeed 28, they probably rode the tar out of him for a day or two, tired him out totally so he might have been anemic or just flat wore out, 2 weeks of food and he was back to his nasty self. THAT happens all the time.
I will not let a horse go on trial ever. Like Faye said, they come back used up, hurt, and not the same horse. I know a person who let a horse go on trial for two weeks, the "buyer" used the horse on a very long long hard trail ride/hunting trip and then gave him back, said he wasn't working out. Found out later, the guy wanted to go on the trip, didn't have a horse, so worked up the scam and had his trip.
As far as misrepresentation, unless a contract is signed as to exactly what the horse is, does, etc, its pretty much "buyer beware" . If you hand over the cash, you own the horse.
I welcome folks to come try the horse over and over, I will even haul the horse to another area for them to ride (they pay gas) without a problem.Trail rides, arenas, no problem. But, they will not take the horse out of my sight unless they have bought it.