Yes you can call Marriott and have them make the reservation for your friend using your points. They will note in the reservation that the stay is a gift from you. I do this all the time for my wife and kids. I am sure there are those that may be suspicious that this is a way of selling points but I have never had any trouble and it may help that I have always done it with people who share my last name.
were you using the generic Marriott reservation number, or a specific one based on your loyalty level? just curious.
and much appreciated for sharing your experience