Where is the previous information you mention where the owner disclosed an unsoundness? Was it in a sale advert? If so it could go against her if she recognised it then but concealed it when you came along
I can't think of any pain meds that would last that long but maybe the horse has a more minor unsoundness that leaves it OK for light work but aggravates if it does anything more strenuous like jumping.
You need to see a legal person really
I had a young horse that totally destroyed its back just playing in the field, they do stupid stuff and you can't be watching them all the time