What should the girl do about her fiancé who denied breaking her hymen after their repentance, claiming he did not see blood despite seeing a piece of skin from the hymen, especially since he openly denied it before people?

You both committed a great sin by engaging in zina (fornication/adultery), and this disharmony is a consequence of your actions. It is incumbent upon both of you to repent, seek forgiveness, and perform many good deeds, based on Allah's saying: "And indeed, I am the All-Forgiving to those who repent, believe, and do righteous deeds, and then are guided" (Qur'an 20:82), and "Indeed, good deeds do away with evil deeds" (Qur'an 11:114), and the Prophet's (peace be upon him) saying: "And follow up a bad deed with a good deed which will wipe it out." Publicly disclosing a sin is another sin; you should have concealed it.

The advice for the questioner is to resolve the problem calmly, either by marrying him after his repentance and her waiting period (istibra') is complete, or by breaking off the engagement if he insists on the sin or continues to tarnish her reputation. If she is sincere in her repentance, Allah will compensate her with a better man.