The parent should try to understand the bully's perspective, every bully has their own past, and commonly it's a dark one. Understand them, let them feel the love since some of them are in a need of one. Let them talk out their past, if they don't want to or they are not ready, give them time. Don't force them to it since that makes the situation worse. Refrain from being harsh, sometimes bullies are quick or hot tempered.