John was taken completely off guard by what he saw when he finally found the source of the disturbance - he started running toward them immediately, drawing his wand before he'd even really registered what was happening. If they were Muggles, he could have them Obliviated later, but the most important thing was to stop this, whatever it was, before anyone got hurt -
But a second later he realized they weren't both Muggles. He'd have recognized Avery's face from the posters after the Azkaban breakout even if they hadn't been at school together. He stopped a couple paces away from them, wand aimed squarely at Avery's chest, refraining from cursing him then and there only out of deference to the poor woman he had struggling in his arms. "Let go of her," he said, trying to decide whether he had time to put up a signal to call for assistance. "Now." Avery was a good bit bigger than him, but he might be able to take him in a duel - and he didn't really relish the thought of sacrificing his aim even long enough to send for backup when the man had a hostage.