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.