You shouldn't focus on one girl to begin with, and the only reason you should lack that much respect for another couples relationship to try and break it up is because you're a lil' bastard (you lil' bastard you!) looking for a challenge.
You want to break up their relationship because she might like you? Because she is the only girl that reciprocates real conversation back when you guys talk? (sorry if that's offensive but that actually happens)
But if you want some advice... erode the relationship away from within. How? I don't really want to elaborate or spoon feed anybody that sort of information.
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