Praise be to Allah.
What your relative must do is repent to Allah, may He be exalted, from this grave sin, and offer a lot of prayers for forgiveness, and regret it. He should also increase his righteous deeds, in the hope that Allah will accept his repentance. For zina is a major sin, and because of its abhorrent and repugnant nature, Allah, may He be exalted, has ordained the hadd punishment for it, either flogging or stoning.
But by His mercy towards His slaves, Allah, may He be exalted, causes sincere repentance to erase whatever sins came before it.
Allah, may He be exalted, says (interpretation of the meaning):
“And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty.
Multiplied for him is the punishment on the Day of Resurrection, and he will abide therein humiliated –
Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful” [al-Furqaan 25:68-70].
And He, may He be glorified, says:
“But indeed, I am the Perpetual Forgiver of whoever repents and believes and does righteousness and then continues in guidance” [Taa-Haa 20:82].
The one who committed zina is not obliged to marry the woman with whom he committed zina, and this is not a condition of repentance. But if they both repent to Allah, may He be exalted, and decide to get married, there is nothing wrong with that.
Therefore your relative should look at the condition of the girl and her family. If he thinks that she is a suitable match for him, and he knows that she has repented and become righteous, then he may pray istikhaarah, asking Allah to help him to decide, and marry her, because that will be showing kindness to her, and he is the most appropriate person to show her that kindness, because even though she committed a bad deed and sinned, he was her partner in all of that. Perhaps she did that because he called her to do it and tempted her, so he should bear responsibility, with her, for what they did together. In fact, even if he was not the one who committed zina with her, but he knows that she has repented sincerely, and he wants to marry her in order to help her remain chaste and to conceal her deed, that is a noble aim for which he will be rewarded, if Allah wills.
The Messenger of Allah (blessings and peace of Allah be upon him) said: “The Muslim is the brother of his fellow Muslim; he does not wrong him or let him down. The one who meets the needs of his brother, Allah will meet his needs. Whoever relieves a Muslim of trouble, Allah will relieve him of one of the troubles of the Day of Resurrection. Whoever conceals a Muslim, Allah will conceal him on the Day of Resurrection.” Narrated by al-Bukhaari (2442) and Muslim (2580).
Al-Haafiz Ibn Hajar (may Allah have mercy on him) said:
The words “or let him down” mean: he does not leave him with someone who will harm him, or in a situation in which he will be harmed; rather he helps him and defends him. This is of a higher degree than simply not wronging him; it may be obligatory or it may be recommended, according to different circumstances. Regarding the words “The one who meets the needs of his brother”, in the hadith of Abu Hurayrah narrated by Muslim it says: “Allah will help a person so long as he helps his brother.”
The words “Whoever relieves a Muslim of trouble” refer to a stressful situation that overwhelms him.
End quote from Fath al-Baari.
If the woman has repented from zina, she does not have to tell the one who proposes marriage to her about her virginity; in fact, she does not have to tell him even if he asks her, because she is enjoined to conceal her errors, and hymen may be broken not only by zina, as it may be broken as a result of extremely heavy periods, jumping, and the like.
And Allah knows best.