Is it a correct action for a suitor, who proposed to another woman at the same time and then left her after describing her as not sensible and easily surrendering herself, to be rejected? And is it permissible for a young man to do that?

It is not permissible for a suitor to go out with his fiancée for a stroll, because he is a stranger to her (non-mahram), and his doing so necessitates repentance. It is not advisable to reject a suitor directly; rather, one should inquire about his religiosity and uprightness. If he is upright, then she should seek Allah's guidance (istikhara) and proceed with the marriage; otherwise, it is better to leave him.