Cardholder does not need to be traveling with the companion ticket. However, the ticket needs to be booked form inside the cardholder’s frequent flyer account. (and technically paid for with that card though there are assorted data points of people who were able to use other AmEx cards to pay) This is to allow for legit friends and family uses of the cert while discouraging it from being bartered or sold, which is against the T&Cs.

IIRC, you must book it as a two person PNR for it to work.