Why have some of my samsung galaxy grand neo plus contacts appeared in a friend's phone?

[AC Question]

My friend has recently purchased a samsung grand neo plus mobile phone. Yesterday some of the contacts from my samsung grand neo plus phone appeared in his contacts list. These are contacts he does not know. He has never phoned them. They have never phoned him. At the time these contacts appeared he had just completed installing Viber. The contacts are in my Viber contacts list. Their photos appeared too. We share the same WiFi in the same house but have different Gmail addresses on our phones. Can you offer any opinion as to why this has happened?