I would not risk trying someone else's number. Even if you somehow get it to stick, you can get into trouble if they catch you using it for free bags - they can bill you (and possibly fine you as well), and if they're in a really bad mood, they can target your mileage account as punishment, as well as report to the airline of your friend's account that it's being used fraudulently, and get him shut down as well. (I know someone who somehow got someone else's number to work, then got caught - he got into huge trouble for it. I don't remember exactly what happened to him, but I remember he said it was totally not worth the free bags.)