Is a girl's oath not to marry after the death of her former fiancé considered a correct action, or is it a mistake for which she should be advised to overcome it based on the Qur'an and Sunnah?

Marriage is a tradition of the Prophets, and they had wives and offspring, as Allah Almighty says: ﴿And We have certainly sent messengers before you and made for them wives and offspring.﴾ Abandoning marriage goes against the innate nature upon which Allah created mankind, and it leads to falling into the unlawful.

Therefore, the girl must marry, and she is not obliged to fulfill her promise to that young man. As for her oath, she should expiate it with an oath expiation, for the Prophet, peace and blessings be upon him, said: "Whoever takes an oath and then sees something better than it, let him do what is better and expiate his oath."