Is the relationship that occurred between the girl and the deceased man, in which marriage was agreed upon without witnesses or a guardian, considered a legitimate marriage, and what is the ruling on her marriage to the young man who proposed to her, who fears that he might be a cuckold for accepting to marry her?

One must repent to Allah Almighty from zina (fornication/adultery). Sincere repentance involves regretting the sin, abandoning it, and resolving never to return to it. Repentance erases sins and transforms bad deeds into good ones. One who repents from a sin is like one who never committed it. A fatwa (definition) that permits zina is void and contrary to Islamic law. One should have concealed the sin and not divulged it. There is no harm in a man marrying a woman after her repentance, nor is he considered a pimp (dayyooth).