Is what the betrothed couple does, spending a holiday night together in the fiancée's room, considered adultery, and is this the reason for the delay in their marriage?

If a woman has an intimate relationship with her fiancé as if they were spouses, without a valid Islamic marriage contract, then both of them are committing fornication. They must repent and perform istibra' (ensuring no pregnancy from the unlawful act) before marriage. They must abandon the sin, regret it, and resolve never to return to it, while keeping it private and not publicizing it. If the woman is also neglecting her prayers, this is a grave sin that necessitates advising her and warning her of the consequences of abandoning prayer and committing immoral acts. If she does not repent from these major sins, then the relationship with her must be severed, and one should associate with righteous women.