All praise is due to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
The basic principle in choosing a husband is to consider his religious commitment and character. The Prophet ﷺ said: “If someone whose religious commitment and character you are pleased with proposes to you, then marry him.” Therefore, the mere fact that a man is a mujahid and is preparing to go to battle is not, in itself, a valid Islamic reason to reject him, provided that he is sound in his religious commitment and character.
Moreover, going to jihad does not necessarily mean that a person will die, just as remaining at home does not prevent death. Lifespans and appointed times of death are decreed by Allah, the Exalted. He says:
“When their appointed time comes, they can neither delay it by a single moment nor bring it forward.”
And He says:
“No soul knows in what land it will die.”
Therefore, it is incorrect to believe that a person's death is near simply because he goes to battle, or that he will escape death if he refrains from doing so.
Accordingly, the father's fear that his daughter may become a widow because her husband might die in battle is not, in itself, a sufficient Islamic reason to reject the suitor, as long as his religious commitment and character are sound, especially since death can come to a person anywhere.
Nevertheless, a woman may not marry without her guardian's involvement. The Prophet ﷺ said: “There is no marriage without a guardian.” He ﷺ also said: “Any woman who marries without the permission of her guardian, her marriage is invalid.”
She should therefore deal with her father wisely and discuss the reason for his refusal with him. She may also seek the help of knowledgeable and sensible members of her family to persuade him, if the suitor is satisfactory in his religious commitment and character. However, if her father has a valid reason concerning the suitor's religious commitment, character, or ability to fulfill the rights and responsibilities of marriage, he may reject him on that basis.
In conclusion: A righteous suitor should not be rejected merely because he might die in jihad, for the appointed time of death is in Allah's hands; going to battle does not bring death forward, and staying away from battle does not delay it. However, a woman's marriage is not valid without her guardian, so she should handle the matter wisely and seek to persuade her father through lawful means.
And Allah knows best.