Legally, she doesn't owe him anything.  He was a licensee on her property.  She owes him whatever their contract says she owes him and nothing more.  If I were in her position I would be inclined to give him a finders fee, but I don't know whether it would be more than 10%.  He did "find" it, but not on his own volition.  Opening up the wall was, presumably, her idea.  His honesty is commendable, but if I drop my wallet and some nice person picks it up and says, "Excuse me sir, you dropped this," I don't think that he is entitled to nearly half my money just because he could have stolen all of it.