Agreed there are always 2 sides to a story!
But at the end of the day, the op hired a professional. Who's job is to anticipate the possible interference from a third party, be it another trade person, a tenant, a minor or home owner that does not realise or know not to top up the pressure. A true professional would have made a quick risk assessment and eliminated the possibility of this happening with just a few good practice measures which lets face it, should always be carried out regardless of the property being occupied or not just to protect the property, decoration & contents. In my mind the plumber is guilty of negligence and needs to be brought to justice. Not try to blame a faulty filling loop and try to make more money out of it. Because even if the valves were faulty (which I don't believe) but given the benefit of the doubt, he still should have followed simple procedure and the flooding would not have happened faulty valves or not.