Problem with Apple ID

I recently changed my Apple ID and password after I noticed that a friend's (who used my old Apple ID to download apps on his iPad and iPhone) contacts has been synced to my iPhone contacts. Even after I changed my Apple ID, his contacts are still getting synced plus I've noticed there are apps getting purchased in 'not on this phone' section on iTunes. So, I changed my Apple ID and password yesterday with a new email account. I'm so confused. Is there a possibility that he can still use my new id to purchase apps? and through my old Apple ID, could he be able to access my iCloud back up before I changed into my new ID?