I don't see any faults in Nitu according to what's mentioned in the story. Nitu asked for help, Bipin helped her. But that does not mean that Nitu is now obligated towards Bipin. It was Bipin's bad luck or stupidity that she started seeing some other guys before he had the courage to ask her out. So Bipin did what he felt like, that is help Nitu, and Nitu did what she felt like, that is accept the help and fall for some other guy. Moral of the story is, "Key garnu mangaley, afnai dhangaley."