I bought a dog through an advertisement for £400. The animal was described as being suitable as a pet or for showing, I was told that the dog was used to children, but within days the animal bit one of my grandchildren so badly that he required stitches. After much argument the original owner took the dog back, but she wouldn’t refund the money.
Since the dog was sold on the basis of it’s suitability as a pet, you should have reasonable prospects of winning a case for the return of your money in the small claims court. I presume the doctor would be able to supply supporting evidence and your friend could produce a copy of the advertisement. If you could show the animal had bitten previously you might be entitled to more money by way of compensation.