Why would a girl who knows a guy watch his girlfriend and try to be around her? She tried to record his girlfriend? Whats her problem?

She won’t say anything and she only watch the girl. She follow the guy on Instagram and both went to the same high school. He also communicated to his girlfriend mainly when this girl comes around his girlfriend job. He told his girlfriend he was in a certain that other girl vists.

Why would a girl who knows a guy watch his girlfriend and try to be around her? She tried to record his girlfriend? Whats her problem?
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