So If she sends you out a bill for the other plumber pay it in full. Sometimes things just don't go the way you would like. I don't see this going to far. Maybe a cleaning bill, and the other plumbers bill. You might try to send out a letter or go yourself to explain the situation. Talk to the lady that called you, to see what she may be able to tell you, about the other neighbor, and what she wants, to make her feel whole. You may want to eat this whole job. and send out gift card for a restraunt meal, to get this all behind you. Good luck.