This does smack of insecurity. There will always be people who take a liking to people's boyfriends/girlfriends and we can't keep tabs on them all. That's why we have to trust our partners. Is your friend a slightly insecure person? I mean, even if the girl in question had announced, "I like your boyfriend and I'm going to steal him from you," I wouldn't be too impressed if my girlfriend didn't trust me enough to go and warn her off behind my back and would only push me away from my girlfriend. Surely the trust built up between her and her boyfriend is too important to just throw away? The real person she needs to talk to if she feels threatened is her boyfriend, who if he's worth his salt, will reassure her as long as the emphasis is placed, not on her fear her boyfriend will cheat, but more warning him about the stuff this girl has said. Don't go behind his back...