If a prostitute converts to Islam is she forgiven? And what about a man who marries a prostitute and converts his wife to Islam are they both forgiven?

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 is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Allaah Says (which means): {Say, "O My servants who have transgressed against themselves [by sinning] do not despair of the mercy of Allaah. Indeed, Allaah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."}[Quran 39:53]. Furthermore, Ibn 'Abbaas narrated that some polytheists, who killed too many lives and committed fornication and adultery too much, came to the Prophet and told him: "Indeed what you are calling to is good, but could you inform us if there is any thing that we can do to expiate our sins?", then the following verses were revealed: {And those who do not invoke with Allaah another deity …}[Quran 25:68], and the verse {Say, "O My servants who have transgressed against themselves [by sinning] do not despair of the mercy of Allaah. Indeed, Allaah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."}[Quran 39:53].

Besides, the Prophet informed us that Islam wipes out previous sins as he said: "Are you not aware of the fact that Islam wipes out all the previous misdeeds? Verily migration wipes out all the previous misdeeds and verily the pilgrimage wipes out all the previous misdeeds." [Muslim] Therefore, whoever embraces Islam, Allaah forgives him/her all his/her previous sins whether they are minor sins or great major sins.

On the other hand, if one is the cause for a person becoming Muslim, he will be rewarded for this act, as the Prophet said: "Whoever, initiates (revives) a good deed in Islam he gets its reward, and the reward granted to anyone who does it after him without reducing the reward of the doer himself."  [Muslim]

You have to know that the non-Muslims or disobedient Muslims who repent to Allaah, Allaah will accept their repentance and forgive them. Allaah Says (which means): {And it is He who accepts repentance from His servants and pardons misdeeds and He knows what you do.}[Quran 42:25].

Finally for more benefit, for the conditions of repentance, please refer to Fatwa 87903, and Fatwa 81160 on the ruling of marrying a prostitute, and Fatwa 85425 on the dangers of fornication and adultery.    

Allaah Knows best.