Catalytic Converter Failure

We took our 2003 Ford Escape in for a slight oil leak (4 tiny drops on the garage floor) Mechanics said they found a cracked oil pan and some gaskets wearing out. They replaced that and performed a tune up since the engine was open. They said the engine was missing after the repair. We picked it up and it was very rough riding. Within a few miles of driving it died. Now we are told that 2 catalytic converters melted and that was the problem to begin with and not their problem. They refuse to fix it. The engine was not riding rough nor was there an engine light on until after the repair. Could they have done something to cause this problem with the catalytic converters?