How is the evidence strong? The evidence here that he has anything to do with it is just the family saying "she wouldn't do that". It could have been anyone in the club that did it, or it could have been her own drugs.
Even if they did find the guy all the police can do with the zero evidence they currently have is to ask him if he gave it to her. He says no (even if that is a lie) and walks out. The police are well aware that any further investigation is a waste of money and just puts a target on the guys back for potentially no reason.
Ahh my bad!