Is the suitor sinful if he does not officially marry the girl and then divorces her, or if he does not marry her at all after their customary marriage, and what should he do?

An unregistered marriage (zawaj urfi) without a wali (guardian) is forbidden and invalid according to the majority of jurists. The person who performs it is considered an adulterer if they are aware of its prohibition, based on the Prophet's (peace be upon him) saying: "There is no marriage without a wali." However, if a ruler or judge ratifies it, their ruling is not to be overturned. If the contract was not made with a wali or his representative, and no Muslim judge has ratified it, and it has not been publicized, then it is to be annulled, and the couple separated. You are not obligated to marry this woman, but if you can try again with your mother and marry her in a valid marriage, not with the intention of divorce, then it is an act of kindness and a covering for her, and she must repent. Temporary marriage or a marriage conditioned upon divorce after a period is impermissible and invalid. If marriage to her is not possible, then stay away from her and beware of the steps of Satan, and increase your supplications for forgiveness and righteous deeds.