Account bound armor and weapons can only be used by the person they dropped for. Though I haven't tested it, allowing transmutation stones from someone else to be used on it would pretty much make something being account bound worthless. These things are account bound for a reason.
Indeed if it is Account Bound only that person's account can do anything with it, just like Soulbound only 1 of your toons can do anything with it. However.... I believe your friend can transmute the skin onto another weapon which is not bound and you can then get that weapon and transmute it to the one you want. I HAVE NOT tested this. But it has occured to me because I have done a similar thing with soulbound trading the skin of a weapon that was soulbound on 1 toon to a cheap weapon I then transmuted to the weapon I wanted on my other toon. So in theory this could work with account bound as well.