One of the worst things about petrification was the fact that the body was completely frozen, but the mind was fully function. dean could see and think and what he saw was absolutely heartbreaking. The flash of green was unmistakeable, even if he didn't hear the accompanying words that ended Mrs. Finnigan's life. But he couldn't act. He couldn't yell, or do anything to stop or distract the witch that had just killed Seamus's mother.
But then he saw what she did next and that too was familiar. It had been in Ron's memory. The box. The spot he didn't know Mrs. Finnigan had disappearing, in a black whisp into the box. Just like Penny.
Seamus's mother was dead and Dean couldn't save her, and now he had to wonder what the witch's plans were for him.