Listening when he was searching for a mind was something that couldn't be helped. Not unless he were to break contact completely. Which would have nullified the entire point of this little experiment.
Cordelia seemed on a roll when Peter first found her, however, and so he waited, holding back in order to let her work. It was more an indication of her strain and anger than a lull in the argument, but in any case, it was the best time he could think to interrupt.
He did so carefully, gently letting his presence nudge her mind, with the mental equivalent of clearing his throat.