I had the same situation a few weeks ago. Res was in my name through FHR at RC Georgetown.

I couldnt make the trip so I gave my wife the Plat Amex and told her to use it on check in. She did with no problems. She has her own gold card which I said to use if there were any issues. When checking out she just said leave it on the amex and there was no problem.

At another property a few months ago I booked a room that I shared with someone else through FHR, at checkout we wanted to split the bill and the front desk said that was fine but the other person's half of the bill had to be charged to an AMEX card. So they put it on their green amex, in their name and they are not a PLAT card holder.

Despite what Ts and Cs say, their only major concern is that the room and tax get charged to an AMEX card.