In general, both should defend each other, except if someone is doing something illegal. Like I would not defend a girlfriend who did illegal drugs. I wouldn't say things to get her in trouble, nor would I defend her. She's on her own in this case.
In this case, it sounds like you may have overreacted in the past, so your bf is right in assuming you overreact now. If you want to be treated honorably, you have to act honorably. And it does sound like both of you girls have some jealousy for each other. So, she should not be "bumping" you. I know this game, it was done to me and others in high school. It's a passive-aggressive move for cowards who don't want to address a situation. This is a bullying tactic, and bullies only understand violence. So you might have to push back, I don't know what will work.
If you want to push her back and play games, you can, but it will subtract from your reputation.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
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