Bop-
several of my son's friends made contact through letters or visits while he was in rehab. it did not help him and probably made his friends feel helpless -
you are right, there is not a lot of information out there about friends of addicts, or I haven't seen it. But looking at the advice about families dealing with addicts applies to friends also - depending on your friend's addiction and possible recovery or not - he could ask you for money, he could steal something from you while in your place, he could break into your place if he things you have something of value...
when I say that his friends letters/visits didn't help him, it was all on my son. His friends were loyal and great - he just was in no frame of mind to appreciate or understand their intentions.
thoughts, prayers, sending good karma - you can always do this.
b.