If it looks like a duck and walks like a duck.......................
I would hesitate to jump all over the vet, the owner is not feeding the old horse enough food, by her discription of her feeding regimine, hence feed it more. I would balk the money involved with a battery of tests before I put more groceries in the old horse to see if that was the problem. Which I would guess is the problem.
Plus the entire conversation with the vet was probably not divulged in the OP, since the OP was just asking an opinion of, is this horse thin.
I don't disagree with any of that
I think, in today's world, many vets are too worried (or maybe they are taught in school) about being politically correct.
They leave things like "he's getting thin" as a dangling participle rather than discuss the feed program with the owner and also the possibilities of running tests.
When a horse is 29 running tests, understandably, might not be something an owner wants to invest in but, the option (I think anyway) should be discussed by the vet with the owner in detail.
My vets are good about speaking up but I still look them square in the eye and tell them to just "spit it out", whatever "it" is and don't mince words with me.
I've been at this too long and I'm far too old to be molly-coddled by namby-pamby unfinished thoughts out of anyone's mouth.
Unless there was more to be said, that's why I said I would smack the vet; if he felt the owner needed some diet guidance then he/she should've offered it up ----- in a lot more gentle way than I am capable of - lollololl