Accused with Possession of stolen goods that the offender does not possess and does not admit ever possessing

I have a friend that is being harassed by the police. The police say they caught him buying a stolen iPad. He says he never bought one, but the description of the incident is the same as when he once bought a stolen macbook (which he no longer possesses). The officer says it's just a matter of time until they arrest him for possession of stolen property. They keep telling him to return the property but how does he return what he does not possess? I want some legal advice on what would be best to do in such a situation. Thank you.