Officially you can transfer only to your own account or the account of someone who holds a supplementary card. However, in practice if you do everything online, speaking to no one at American Express or the other program, there is a high probability that a transfer will succeed.

If not, you could obtain a supplementary card for your friend. Bear in mind the card only needs to be issued and activated, it does not need to be used, which means you do not need to physically give the card to the other person if it is not appropriate in the relationship. Depending on the card you may receive a bonus offer for ordering supplementary cards, but there may also be an annual fee.