Is it obligatory for someone who committed fornication in the past and fathered a child to inform his fiancée or her father about it, or is it permissible for him to conceal it, and would concealing it be considered deceit or lying?

It is not permissible for a man to inform his fiancée or anyone else that he committed a lewd act or that he has a child born out of wedlock. This is not deception; rather, it is his duty, along with repentance and concealing his sin. Whoever falls into disobedience must repent immediately and conceal himself, for it is stated in the noble Hadith: "Whoever commits any of these abominations should conceal himself with the concealment of Allah." If he is compelled to lie to conceal himself, there is no sin upon him. Among the permissible instances of lying is to conceal one's own sin or the sin of another, as long as the other has not openly declared it.