My chunky east keeper horse will drop noticeable weight in a heartbeat with any significant trailer ride. Just how he is. And the 5yo I imported last year showed up looking like a starving greyhound after that travel which was not especially long or difficult as imports go (and I had been to the place where he came from and was bred and raised and know how they care for horses plus saw video of him getting on the trailer when he left). He was also wormy and needed dental work and I’ve been around enough to know there’s a lot of variation in how fastidious people are about that kind of care.
And there are sooo many cases where 1 person says ready to show tomorrow that person 2 would totally not agree. If you like the horse and want to keep him, I would not rock the boat. I don’t think you are looking at a fraudulent situation, and a lesser fight won’t be worth it IMO.