How to negotiate when you discover old water pipes after opening up a wall during a renovation?

Let me share a down-to-earth experience. During the demolition phase of an old house bathroom remodel, we opened up a wall and found that the old galvanized pipes were severely corroded. The original quote only included replacing the fixtures. The client felt this was something the contractor should have foreseen. The situation on-site wasn't out of control at the time, but letting it drag on…