Sam
Who says its permitted for single men and women to exchange greetings on the street? Even if such a krum pritusdig (yes its krum and pritzusdik for single men and women to stam wish each other good shabbos) minhag took hold in a community its a minhag shtus and one need not abide by it. In all honesty he shouldnt even be looking at them and if he is looking down they very likely wont bother wishing him a good Shabbos because hes a frumie so whats the point of flirting oops I mean wishing him a good Shabbos.