Why would a guy dump a girl because of his bestfriend?

So my friend and I have been trying to figure out why her boyfriend dumped her. We figured out it's because his bestfriend made him do it and his bestfriend is an asshole in general but why would he make him do it? What could be a reason for this cause? Also, what should she do now? Should she progress to taking revenge?
Take revenge. He broke up with you because his BESTFRIEND told him to do so.
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Bad idea.
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Why would a guy dump a girl because of his bestfriend?
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