You need some proper legal advice if you are a gold BHS member you get free legal assistance .. otherwise it may cost you more for the advice than half the vets bill. Sounds wrong to me but I’m no legal expert
You have to prove that the owner of the horse lied to you. How you go about that when it is clearly your word against hers is not known.
Or if you had a vetting you can go down the route that the vet should have know that there was an issue but they can come up with a million and one excuses why something wasn't evident on the day or that they did the checks necessary but nothing showed up on the day.
when I spoke to BHS legal they said that if I wanted to sell said animal I would not have to declare a thing to the potential purchaser and it would be up to them to ask any questions. So asking a general question like 'have you had the vet out to this horse in your ownership' may not cover it.
It was basically 'does this horse have an issue with his hooves?'. Does it have an issue with its pastern? Does it have an issue with its fetlocks? Does it have an issue with its cannon bones. Does it have an issue with its knees and so on and so forth until you have covered every bone in its body. Then you can move on to skin. Then behaviour. In about two days you may have covered everything. And then its still your word against theirs.
This was also confirmed to me by an equine solicitor.
Apparently due diligence is a lot more complicated. Asking on dodgy dealer websites/word of mouth/getting a five stage vetting/asking the right questions/taking someone with you IS STILL NOT ENOUGH