Alice and Bob both live in a property which is divided into 2 flats. They are jointly responsible for the well-being of the building as a whole, but their respective flats are individually owned.
Bob asks Charlie to do some building work for his flat. Charlie does so. Also, another builder, Derek, has been contracted to do some general maintenance work on the building as a whole.
Alice finds that part of the exterior of her part of the building has been damaged when it wasn't before. She asks Derek about it and he says that Charlie did it. She therefore asked Bob to pay for the repairs for her part of the building.
Though Bob cannot be absolutely certain, he thinks that it is probably the case that Charlie did indeed cause the damage.
What should Bob do (both morally and legally), and why?
...or just paranoid?