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Mary never should have turned her back on the hell beast. She was a Hunter; she should have known better.
As it was, when the creature reared up in the Impala's headlights and lunged for the human woman, the instincts Shadow had been suppressing since her arrival on Earth came to the fore: she transformed, one hand sweeping Mary safely aside, while the other shifted into a blade that pinned the beast briefly before slicing it in half.
Then Mary shot her, rock salt powdering harmlessly along her side.
Three exorcisms and a silver bullet later, Mary was finally willing to listen. Shadow could tell Mary didn't particularly believe her, but she was willing to listen.
And eventually, when she transformed back into her vehicle mode, Mary consented to put the guns back in the trunk and drive home with her.
"John never finds out about any of this," Mary said, her hands white-knuckle tight on Shadow's wheel. "And you have to swear you'll never do...that...again."
"I promise."
The next morning, when she came out of a particularly heavy recharge to the itch and sting of freshly engraved runes on her plating, and a curious blank spot where her transformation protocols ought to be, she knew Mary hadn't quite been willing to take her at her word.