They can add your name in the booking so that you can check in, you tell the front desk that your Diamond friend will be "arriving later". If this is in the US, you could also pretend to be your friend as hotels don't ask for ID, though this is fraudulent. Or even better if the hotel has digital key you might not need to check in physically at all - but your friend would need to trust you with their Hilton login credentials. All these would get you the benefits.