Why does my friend iphone 6 calls show on my phone
The Apple ID is how the iCloud system identifies you. If you share an Apple ID, then iCloud thinks you are using both phones. If you don't want the calls to show, then don't share the Apple ID.

Why are you sharing an Apple ID? Is it to be able to share purchases from iTunes/App Store? If so, you CAN use a different Apple ID for the App store than your Messages, iCloud, backups etc. Simply use the shared Apple ID with the App Store login, but for iCloud, use a personal Apple ID. That way, calls won't show up on your phone, but iTunes/App Store purchases can be shared. Or, use Family Share, as Family share allows you to include personal Apple IDs in a purchasing 'group'.