A320 flight crew refused an aircraft; which had flown its previous leg with an improperly resolved logbook item.

Narrative

My First Officer and I arrived with a 40 minute turn to another A320 parked next door. We soon found out we had a plane change. Enroute to our new gate my First Officer ran into another crew who had just gotten off Aircraft X. They were going to the plane we were taken off. We were going to Aircraft X. The Captain explained to my First Officer that they had brought Aircraft X in and were supposed to fly it on; but had refused it. They explained that the engine 2 HP valve fault had been written up. Per MEL; Maintenance Control had told a contract Mechanic to secure the valve closed and 'inop' it. In doing so the Mechanic had broken off the head of a bolt that is used to safety wire the head of the valve closed. Somehow the plane was released to fly to its destination; but the flight crew then decided they did not want to take it any further. When we got to the airplane there was no reference anywhere to either their refusal or the broken unsafetied bolt anywhere in the paperwork. I called Maintenance Control and he said he would have a company Mechanic check it out. That Mechanic confirmed that the bolt head was broken and not safety wired. He said the bolt could have worked itself free and the valve would have opened. He said he would file a safety report and I told him I would file one too. At that point I refused the aircraft and we were given another aircraft. My concern is that this aircraft ever flew out in that condition and also if not for the coincidence of my First Officer bumping into the other crew no-one would have known until eventually the valve would have been changed.