Proof of dog ownership in uk?

my step daughter has just split up with her boy friend and moved back in with us and her dog that he bought her now he wants the dog back he only has the papers with the breeders name on it but she has him registerd at the vets in her name and she had him taged he allso kept the dog caged all day and beat him she is desperate to no were she stands can eny one help asap