As I said, "you would both hold joint title over the property, subject to (1) the true owner and possibly (2) the landowner (depending on the state you live in)." It's not really about a defense, it's about who has proper title over the object. If you both find it, then you have joint title. I suppose the only defense would be that you found it first, and the other person was not involved in the finding. If you claimed that your friend was simply a 3rd party, rather than a joint finder (haha) then you would have better title over your friend.