What should a girl do with her fiancé who non-Islamically deflowered her, then denied the incident despite seeing a piece of the hymen, and announced his denial before people?

The jurisprudential ruling on the issue of hymen rupture varies depending on whether it occurred before or after the marriage contract, and whether it was through illicit sexual intercourse (zina) or other means.

If it occurred before the marriage contract through zina or by finger, you have both committed a grave sin from which repentance is obligatory, and you are not entitled to a dowry since there was no coercion.

If it occurred after the marriage contract through intercourse or by finger, and the husband claims that you are not a virgin because he did not see blood, then his claim is rejected, as bleeding is not a condition for virginity. Even if the hymen was removed due to a cause other than intercourse, he has no option to annul the marriage. If he stipulated virginity and claimed that she was a non-virgin (thayyib), and she claimed that her virginity was lost due to his intercourse, then her word is accepted with her oath. If he did not stipulate your virginity, then he has no option to annul the marriage, and you are not required to take an oath.