Peter had played the solitary hero and the martyr both enough time to know the signs. And he knew without a doubt that Prue was planning on facing Barbas alone. And he wasn't about to let her do that, not after she stood up to Danko for him. And especially not after she'd shown more concern for him than his own family.
He focused on Molly's power, using it to find her, then teleported to her side with Hiro's. "You didn't think you were going to face him alone, did you?" he asked. "Even if I was cool with that, Chris would murder me if I let you."