I think you can read your boss - if they look pissed that you don't disclose the name or they keep asking - that's pretty plain.

Is Cain responsible for the quality? Yes. But IMO the boss should have delegated handling of the issue to him instead of retaliating for apparent insubordination.

He could legitimately complain that Cain provided insufficient process and controls to prevent junior devs from making critical errors. But looking for a scapegoat is indicative of bad management.