The OP, who believes there’s a cover up and is passing because she doesn’t believe the horse is just sore from nail ons and inquired no further once being given a reasonable explanation (horse was sore from nail ons) and is assuming nefariousness from the fact that she saw a couple other horses in nail ons at the same barn.
She’s entitled to pass for this reason or any other reason. Including just not wanting to deal with a hassle of glue ons. You can pass on any horse for any reason and have my support. Any buyer should feel comfortable. And she found another horse she likes. Which is reason enough. I fully support passing for any reason that the buyer is uncomfortable.
But if I tried a horse, liked it, and it was sound while riding-- and what I was told about glue ons was that nails made the horse sore-- I would take that at face value and inquire into the circumstances (how long has the horse been in glue ones, what was the quality of the hoof when the nail ons were an issue, what farriers are involved, etc.) and not go looking for reasons why it’s got to be something much more nefarious than a horse that, as the owner says, was sore in nail ons and so they switched to glue.