Is it possible for a person to marry someone whom God has not destined for them, and does a fiancé leaving his fiancée mean that God did not destine them to be a married couple?

Belief in Divine Decree, both good and bad, is one of the pillars of faith. Everything that occurs in the universe is by Allah Almighty's decree. There is no repeller of His judgment, no withholder of what He gives, and no giver of what He withholds. What Allah has decreed, no one can avert, and what He has not decreed, no one can attain, as stated in the Hadith: "Know that if the whole nation were to gather to benefit you with something, they would not benefit you except with something Allah had already written for you. And if they were to gather to harm you with something, they would not harm you except with something Allah had already written for you. The pens have been lifted, and the scrolls have dried." Everything has been created with a measure; Allah has decreed everything. A servant will not truly believe until he believes in Divine Decree, both good and bad, and that what befell him was not meant to miss him, and what missed him was not meant to befall him. Indeed, the first thing Allah created was the Pen, then He commanded it to write, and it wrote everything that will be until the Day of Resurrection. Therefore, marriage and separation occur only by Allah Almighty's decree. He has perfect wisdom in all that He decrees, and the Muslim must be content with Allah's judgment and decree.