As long as the horse is N/N then everything is ok. All Impressive bred horses should have stated on their registration papers what they are N/N, N/H, H/H. A large number of horses have an Impressive bred horse somewhere in their pedigree. My dad has a paint horse that back on his damn side is a mare that is related. He also has a little reining mare that also is an N/N. He was a nice looking horse and was a very popular stallion that has offspring in the AQHA, APHA, and in the Appy Breed. The AQHA has just recently passed a rule stating that any horse that is (if I remember correctly) N/H or H/H is unable to be registered with the association. IF the horse is N/N then you don't have anything to worry about and neither should the person who bought the horse. Also, my opinion is that the buyer didn't do his/her homework. I always look at pedigrees when I am buying a horse.
If you honestly didn't know then at least you are now educating yourself. So do you know what the horse is?