Data recovery from iPhone

A close associate of mine has admitted stealing and fraud. He offered to lay down the cards on the table and tell everything. He has granted me access to his iPhone including the phone password. But I have a suspicion that some of the stealing and fraud has been done using snapchat. I hope I am wrong, but want to check if this is true. I need help to recover deleted snapchat messages and media from the iPhone. And though he allows me to check his phone, I would like to do it without him knowing it. Can you help?