Confronting a thief on your own before informing police?

My friend got his wallet misplaced recently, when he forgot it at a cafe and when the owner asked whose wallet it was in the cafe a person came up to him and said it was his. There was no identifiable documents or material in the wallet that could prove if it was his or not, except a debit card. My friend has tracked the person down with his debit card and intends in first asking him online through Facebook about it and if refused go to him directly. He insists on keeping it from his parents for now until the matter is closed and wants to only involve the police if the person refuses/denies. My exact question here is if it would be better to inform the police first rather than later. I don't want him to hurt himself incase things get violent