It does depend on the hotel's practices, and the individual working the desk, but it can happen. See this
example where the hotel would not allow another name to be added to the reservation -and- would not allow the companion to check-in until the cardholder arrived...continue reading to at least post 606.
In that example, he didn't add the second person to the reservation. He tried to add the second person with the hotel, but it wouldn't let him. When I said there's no risk of hotel not honoring the reservation, it is predicated on the name of the person showing up actually appear on the reservation as the second person. In that case, even if the Cardmember doesn't show up, the reservation should still be good. Then it's up to the hotel to honor the FHR benefit.
"To accomplish this, I chatted into the hotel via the Amex chat thing and got no answer for several days. So, today, I called the Wynn just to confirm there would be no issue. They told me my travel companion will have to wait in the lobby until I arrive, and there is nothing that can be done. Because Amex is a third party system, they cannot add the second person to the reservation. I want to reiterate that I have booked and paid for two people, and am just trying to insert a name into the currently blank space."