In my experience, if the Platinum cardholder is traveling with the other person, then both will receive the IAP rate. However, if the Platinum card holder is not traveling with the other person, they will not honor the IAP rate. The workaround is to add the other person as an Authorized User on the account. The savings received via the IAP rate can more than make up for the additional $175 annual fee.
Of course, calling in and being friendly with the Platinum Travel agent is the best first step.