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.
There is no doubt that Pharaoh's wife was a believer as stated in the Quran and the Sunnah. Allaah Says (what means): {And Allaah presents an example of those who believed: the wife of Pharaoh, when she said, "My Lord, build for me near You a house in Paradise and save me from Pharaoh and his deeds and save me from the wrongdoing people."}[Quran 66:11]
Besides, Imaam Ahmad reported that Ibn 'Abbaas narrated that the Prophet said: "The best amongst the female dwellers of Paradise are Khadeejah Bint Khuwaylid (the Prophet's first wife), Faatimah Bint Muhammad (the Prophet's daughter), Aasyah Bint Muzaahim, (Pharaoh's wife) and Maryam Bint 'Imraan (Jesus's mother)." Shu'ayb Al-'Arna'oot said that its chain of narrators is Saheeh (authentic).
Moreover, Al-Bukhari reported that the Prophet said: "…Many men reached the level of perfection, but the women who reached the level of perfection are only Aasyah (Pharaoh's wife) and Maryam Bint 'Imraan …."
As for Aasyah's becoming the Prophet's wife in Paradise, then we do not know any authentic Hadeeth that proves this.
However, At-Tabaraani reported in a Hadeeth with a chain that has some weakness and which was narrated by Abu Buraydah about Allaah's Saying (which means): {…previously married and virgins}[Quran 66:5], that Allaah promised his Prophet that He will marry him to the virgin Maryam Bint Imraan and the previously married woman Aasyah Pharaoh's wife." This Hadeeth with this chain of narrators is Dha'eef (weak).
Also, Ibn 'Asaaker reported that Ibn 'Umar said: "(the angel) Jibreel came to the Prophet and Khadeejah passed by, so Jibreel said: "Allaah is greeting her and giving her glad tiding of a home for her made of gold in Paradise between Maryam Bint Imraan's home and Aasyah Bint Muzaahim's one." But its chain of narrators is Dha'eef (weak).
Allaah Knows best.