I desired to marry a girl studying at the university, whose appearance indicates adherence to religion. As the Prophet, sallallaahu ‘alayhi wa sallam, commanded us; I performed Istikhaarah (guidance-seeking prayer). Then, I consulted some of my acquaintances, the foremost of them being the Imaam of a mosque who knew this girl because his brother is married to her cousin. A friend of mine told me that this girl used to help his sister-in-law to cheat on his brother. He pointed out for me some proofs including:
• The brother of this girl was caught while handing a love letter to my friend’s sister-in-law, from a man whom she had a love relation with before marriage.
• My friend used a phone to record calls and it turned out that the girl, whom I want to engage, collaborated with her cousin to facilitate betrayal.
My friend assured me that he was not lying and swore by Allaah The Almighty that he was telling the truth. He is going to summon this girl together with the man who collaborated in the betrayal to testify before the court.
However, before this, I asked my niece about this girl because they study together at the same university, and my niece spoke well about her describing her as adherent to religion and having good morals.
What should I do in this case?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

You did well when you chose a girl who is apparently adherent to religion. Appearances are indications of what is beyond them.  It is said, “What is bred in the bones will come out in the flesh.” The Prophet, sallallaahu ‘alayhi wa sallam, said: "A woman is married for [either one of] four: her wealth, noble descent, beauty or religion. So, marry the religious one, lest you should lose." [Al-Bukhari and Muslim]

You also did well when you performed Istikhaarah (guidance-seeking prayer, according to the instructions of the purified Sunnah. Allaah The Almighty will not fail whoever seeks His guidance.

As long as you chose an apparently religious girl, performed Istikhaarah, and consulted wise people, then you are following the right way. However, you have to verify the truthfulness of the claims on the delinquency of this girl, and act according to your investigations. If it turns out that such claims are mere rumors and the girl is religious and well-mannered; then, you should seek the help of Allaah The Almighty and go ahead in proposing to her regardless of the rumors. If it is proved that she is indeed an immoral girl, and there are no indications of her repentance, we advise you not to engage her. The Prophet, sallallaahu ‘alayhi wa sallam, said: “Marry the religious one, lest you should lose.” However, if she has sincerely turned to Allaah The Almighty in repentance, it should be known that a person who repents of his sin is like a person who did not commit the sin. All humans commit sins repeatedly, however, the best among them are those who frequently repent.

Allaah Knows best.