There was a part of Castiel fearful that if he so much as looked away from the two of them, that they would both vanish too. So he stayed exactly where he was, letting them have their moment of reunion, but never so much as blinking. Just in case.
When Ben threw a casual grin in his direction he grinned back, even under the horrific circumstances, he was still proud of Ben's attitude.
He was tempted to interrupt, to tell Mary that it was alright, that he had told Ben already. But the truth was it wasn't alright, it was far from it actually, and this was a moment for family. So he stayed silent, letting Ben explain things while he made sure no other threats could get even close.