Does the marriage become void if the fiancée lied to her suitor about not having a previous relationship, knowing that she has repented of her sin?

What the man said was a conditional divorce (talaq (definition) ta'liq) before marriage, and this is a matter of scholarly disagreement. Some scholars say it is not valid, citing the hadith (definition): "There is no divorce for him regarding what he does not possess." The majority of scholars, including Al-Shafi'i and Ahmad, hold that a conditional divorce pronounced before marriage does not take effect. Therefore, your friend does not have to inform her fiancé about what happened, and it does not affect the validity of her marriage.