Is the death of the fiancé, which occurred after the fiancée insisted on visiting him, considered a tribulation from God due to her grumbling and desire to break off the engagement? And is she the cause of that, or was his predetermined time of death inevitable?

The appointed time is written with Allah Almighty. Every affliction on earth and in souls is inscribed in a book before Allah created them. Everything is by the decree and divine destiny of Allah, and man cannot change what Allah has decreed for him. If the victim had been in his home, his destiny would have come forth to him. This is a test from Allah Almighty. Therefore, it cannot be said that the fiancée's unwillingness or hesitation to break off the engagement was the cause of his death.