I own a business and we sometimes get things coming back to us that isn't our fault. How we deal with it very much depends on how it was presented to us. Often, if someone says, look, this happened, I know it's my fault/an accident, we might say we'll fix it at a reduced rate. If the person comes in demanding it sorted out and trying to push it through with accussations/lies, then we'd be much more inclined to investigate it properly and then charge full price if it is determined its not our fault.
Other factors in our response would be, what we would think is a fair thing to do rather than what we are obliged to do, what the goodwill would be worth to us (repeat business etc). I do not know the exact circumstances of the OP, but I would always suggest the more reaonsable you are, the more reasonable the person you are dealing with will be. However, a lot of this depends on the person you speak to. A manager with the authoority is better to speak to as they may be more able to flex the rules.