There needs to be open communication to identify the problem. And focusing on the problem not the person, and attacking the problem not the person. Listening with an open mind, and treating a person' s feelings with respect, and taking responsibility for your own actions. Not walking away from a problem, but trying to openly discuss it and find a resolution. That could go either way, depending on how many victims stood up to them, how many teachers or parents disciplined them, and if they finally realized how much they have been hurting people.
Girls bullying other girls is the big issue now.
I know the school violence with guns and bombs is the most serious issue today, but the most common is girl clicks. Girls tend to talk about each other and hang out in groups where they will ostracize each other. No, they should both learn to stand up to the bullies, girls or boys. That is the first step. Encourage him to resolve it on his own to help improve his self-esteem and listen with an open mind and offer solutions.