Since II doesn't allow exchanges to be rented, contacting II is not a good idea at all. The other person needs to pay for a guest certificate to put the reservation in your name. If you show up with his confirmation, they won't let you check in.
But be aware, that if you get caught renting a II exchange, you can lose the week and the other guy will be in trouble, too.
If you haven't paid him yet, I wouldn't do it. You have a chance of losing all your money and the rental, as well.