Is it permissible for a man to return to his former fiancée, knowing that there is another suitor who has proposed to her and she has accepted him, and then she broke off her engagement to him for reasons unrelated to him? And what should he do in such a situation?

The engagement is a promise of marriage between two parties. If both parties agree and each finds comfort in the other, it becomes forbidden to propose to a woman who is already engaged to another Muslim. If you propose to a woman based on information that she has broken off her previous engagement, there is no sin upon you, even if the engagement was still ongoing, as the mistake is forgiven by the texts of the Quran and Sunnah (definition).

Engagement is merely a promise, and either party is permitted to break it, but this should not be done without a need. The suitor is a stranger to his fiancée until the legal marriage contract is concluded, and communication between them is not permissible under the pretext of getting to know each other.