Why would it make a difference ?
We are talking about adding a 2nd person to the room who has status and having the reservation upgraded to that status.
It does not matter if the diamond member shows up or not as they don't ask both of you for ID.
Ha, thats pretty cool. I had a booking recently where I moved UR points to my cousin's account and made a reservation for him, adding myself as additional guest (i'm only platinum and he was diamond), to get all the diamond bennies.
But it appears from what you say, i could have just booked the room under my name, and added him as additional guest with his diamond status, getting the same benefits. Which would be easier.