Does the suitor bear the sin for the unmarried status of the girl he used to talk to in the past, and what should he do about it?

If you sincerely repent of the emotional relationship, then "one who repents of a sin is like one who committed no sin." You will not be held accountable for that sin, and what happens to the woman is not your concern, nor will you be held accountable for it after repentance. This is with the caveat that an emotional relationship between a man and a non-mahram woman is impermissible.