Cas entered behind Dean and knew Dean could be trusted to keep the weapon trained on the killer and while he was mildly surprised to find that the person in question was female, he disregarded it easily. Johnny's well being was clearly more important and he gestured to Rob to help him with the self-proclaimed vigilante.
Once he was out of the way, he placed pressure on the wound and took a deep breath. Without alcohol or any form of tools, this was going to be more difficult than when he'd done it to himself or Anna.
"I'm going to need you to help me," he said simply. "In case he happens to move, hold him down."
Cas saw no need to explain Johnny's condition. That was his own business and Rob could figure it out himself. And while he personally didn't know how long it would be before Johnny reanimated if he reanimated at all, he could remove the bullet. So that was what he would do. Mary's captor should have considered herself fortunate.