She did something haraam and now a righteous man has come to propose marriage to her but she feels guilty

Praise be to Allah.

Firstly: 

Verily to Allaah we belong and unto Him is our return. This is the result of free mixing and negligence with regard to the matter of forming haraam relationships between men and women. How often has this negligence led us into calamity, regret and sorrow which afflict the one who has lost her honour, then the wolf drops her beside the road because she is no longer suitable for his use; there is no way that he will trust her when she agreed to commit immoral actions with him and he is not prepared to get married at all, rather his goal is cheap pleasure and spending time with the poor, heedless woman.  

Her family will be afflicted by the calamity of losing honour if her relationship becomes known, and every believer in whose heart is any modesty will feel sad when he hears of such stories. The entire society in which immorality and evil are widespread will also be affected. 

Secondly: 

We praise Allaah that you have repented from these evil actions, and we ask Him to accept your repentance and to forgive you, and to divert evil and immorality away from you. But what happened to you is a lesson to others. How often the scholars and daa’iyahs have warned of the fitnah that men face in women, and that women face in men, and have spoken of the importance of adhering to the sacred limits set by Allaah with regard to speaking, listening, looking, shaking hands and being alone with a member of the opposite sex, so that honour will be preserved and chastity maintained at a time when the advocates of immorality are taking such matters lightly and promoting them, and calling them liberation, civility and progress. The matter is as Allaah says (interpretation of the meaning): 

“Allaah wishes to accept your repentance, but those who follow their lusts, wish that you (believers) should deviate tremendously away (from the Right Path)”

[al-Nisa’ 4:27].

It is important that you continue to repent and regret what you have done, and beseech Allaah a great deal and turn to Him, so that Allaah may forgive you and conceal your sin in this world and the next. 

There is nothing wrong with you accepting marriage to this pious man who has come to propose marriage to you, so long as you have repented to Allaah from what you have done. You do not have to tell him anything about the past, rather it is not permissible for you to tell him. Conceal yourself with the concealment of Allaah, as the Prophet (peace and blessings of Allaah be upon him) said: “Avoid these filthy actions that Allaah has forbidden, and whoever commits any of them let him conceal himself with the concealment of Allaah, may He be glorified and exalted.” Narrated by al-Bayhaqi and classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, no. 663. 

Muslim (2590) narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah does not conceal a person in this world but Allaah will also conceal him on the Day of Resurrection.” 

This is glad tidings for the one who repents and is concealed by Allaah in this world: that Allaah will also conceal him in the Hereafter. What matters is that does not persist in the sin and does not take advantage of the forgiveness and forbearance of Allaah, for if Allaah willed, He could expose his sin, for Allaah gives respite but does not forget. 

And Allaah knows best.