Mary's momentarily moment of relaxation ceased immediately when Dean told her he had something to tell her. Being that she was a Winchester, they both were, the chances of it being good news was very rare. Mary allowed him to lead her back over to the bar stools and she reclaimed her seat, keeping hold of his hand as she settled back down. Depending on how bad it was, she might need to squeeze it pretty tightly.
"I saw. I was wondering who he was, but I figured he was just someone you might have met on your travels," Mary replied. "I know you and your brother saved a lot of people, Dean." It wouldn't have surprised her too much if they had gotten close to a few of them. God knows Mary had gotten close to Arthur and while it had blown up in her face, she still knew that attachments were sometimes impossible not to form, especially given stressful situations.
Ben was Dean's son. Dean had a son. Mary had a grandchild. No John and Mary had a grandchild. Mary, who had faced down countless demons and forces of darkness since youth, who had lost everything, actually swayed some on the bar stool with the news. She was a grandmother. She had barely believed herself capable of being a mom sometimes, especially when she returned from the dead and her sons realized she wasn't the mom they thought she was. That she wasn't perfect, especially with cooking.
"You have a son?" Mary took a hold of Dean's arm with her other hand to keep herself steady. It was the one thing that she never thought could make her potentially faint, but it was also something she never thought she would have. "Ben....I have a grandson?" Mary's voice actually cracked and she fought hard to swallow the new lump in her throat.