Yeah, I'm happy to victim-blame Mary for this, which really translates to writer-blame. The whole scene was stupid. For all her avoidance of being Dean's mom, Mary's no idiot. And the writers have spent her entire tenure trying to convince us she's the most badass hunter the world has even seen (even though nothing in her performance indicates that is actually true). So the idea that she'd get all up in possibly-soulless super-powered-Anthony-Fremont's face and practically beg to be whisked off to the Cornfield Hilton was ridiculous. But since the writers made it happen, dumb as it was, I guess it's partially Mary's fault. Of course, as already stated, that doesn't absolve Jack of primary blame. More than one thing can be true.