Crazy!!
The owner of the home is lucky that his friend even bothered to tell him about the money. He could have easily taken the money and not say boo about it. 10% is pretty low, but the owner of the home obviously has more right to it. If the whole lot is worth $500 000, I'd offer him 100 000, maybe 150 000, just to save the cost of lawyers as well as the friendship. These guys will end up spending a shit load on lawyers that they could have just shared and both wind up short one friend.
I forgot he's asking 40%. I'd settle with 30%. Again, if the contractor was just some stranger there'd be a good chance the owner of the home would never have known there was money there.