Is what the young man did with his fiancée considered zina (unlawful sexual intercourse), and is he forbidden from marrying her after what happened?

The suitor is a stranger to his fiancée before the marriage contract, so it is not permissible to be in seclusion with her or to engage in extensive conversation with her without need. Lenience in this matter leads to great evil, such as fornication (zina), which is among the most heinous sins. One must hasten to repent by abandoning the sin, regretting it, and resolving not to return to it, while concealing one's own shortcomings. If you both repent and the woman is not pregnant, then there is no impediment to your marriage.