There is a old saying some mothers use. If your baby bites you; bite them back. (obviously more gently) but you get the idea.
First the subject of how is the child bullying? Is it one incident or has it been many phone calls from the childs school with a long list of complaints from other parents. It makes a difference.
Children do not possess the restraint that an adult has.If they suffer they lash out. And that is what bullies are doing. Expressing their pain and/or frustration the only way they know how.
First emotional issues need to be addressed by the child either speaking with a parent or a counselor/therapist. From there therapy should continue teaching constructive ways to express their feelings.