It would only sync contacts that are in his list and marked as google contacts. If they are marked for the phone, android won't send them to your google account. For instance say my truck is getting fixed at a mechanic and I save his number on my phone. I don't necessarily need the number for the rest of my life so I store it on the phone only and don't sync it with google. It won't transfer to my next phone a year later. I do that pretty regularly.
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