You can buy the ticket with your miles, but you can't put your number on his ticket.

There's no real benefit to him, though if something goes wrong - missed flight, weather delays, etc. - and you've paid for the ticket (whether with money or with miles), then you can try to help out by calling the elite line.

(Not sure that you can't call in just to help a friend, but I'm more comfortable when I've had something to do with the purchase.)