Yes you can go to the DMV and register a car for someone else that is owned by someone else.
You can't register the car in your name though if the title doesn't have your name on it but you can bring the title and have it transferred to the new owner and register it in most states without the owner being with you.
Some states may have different rules though so you would have to check with the actual DMV you're going to so you can find out for sure if they will allow you to register the car for someone else.
You can even renew the vehicles registration for someone else on a car you don't own and all you need is the insurance card and registration papers.
I've registered and renewed a vehicle registration for an older guy who used to live next to be because although he could drive just fine he couldn't stand in line too long in the DMV.
I just took his registration papers, insurance card and the money and renewed it for him and never had a problem doing so.