While Mary might have been expecting Ben's reaction of earlier, the fact that Castiel had broken the news allowed Ben to keep himself in control. Mostly. The look he gave her was serious and a clear attempt at being 'man of the house', but there was a sadness in his eyes that couldn't be hidden.
"I know. But it's gonna be okay, Grandma," he said, patting her back in what he hoped was a comforting way. He had to make sure she was reassured, right? That was part of his job. "Cas promised he's gonna find everyone and we're gonna make them all remember. It's all gonna be okay."
It had to be, it did. Ben didn't even want to consider what it would be like if things didn't go like planned from here on out. They just had to find people, and they always found people no matter how good the bad guys thought they were at hiding everyone. It would be okay soon.