Originally Posted by ijgordon

So what you *could* do is check her in for the flight and get the boarding pass, which would effectively give you access to the two seats. The problem *might* arise when you boarded. You'll need to scan both boarding passes, and that might raise a red flag. It's normally advisable to notify the gate agents at that time that you are flying with an extra seat, otherwise the passenger count will be incorrect before they close the door (if they even still do that.)
While one can normally book an extra seat for comfort (or for large passengers), the extra seat is booked as such, not with the name (and frequent flyer account!) of a non-traveling passenger.
You *might* get away with this on the domestic flight, but it will be more difficult on the international flight, because of passport requirements/checks.

You could try to call Delta and see if they'll effectively change the name on the ticket to have it as an "official" extra seat, but they might. It be willing to do that (for free).