Sometimes you can do a diamond force online as well. Make sure to sign in first before doing your search and sometimes it will say something like "diamond guarantee available" and you can just book that way. If it doesn't work online then I call the diamond desk. Unless your colleague is a diamond, you won't be able to get him a room. T he diamond force only works for one room.

I would also ask the hotel if they have a waiting list. If the hotel just sold out yesterday, you would stand a good chance of clearing the waiting list. Even if you do get a diamond force room I would ask to be on the waiting list since that rate will probably be a lot cheaper and if you clear the wait list just ask them to modify the diamond desk reservation or do a new booking with the hotel and cancel the diamond force room.