My friend is a Muslimah who has been married for the past 2 years. She says her husband treats her well and is a religious man whom many think highly of. However she is overly jealous and keeps asking him things that can lead to arguments - such as (promise me you have never been with another woman) he says no I have not, but she is suspicious and often analyses conversations and re-runs scenarios over and over until she convinces herself that he could be lying to her, how can she overcome this disease as it is causing her much suffering, how can she trust unconditionally? What is this condition called? And what should she do to solve it?

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.

It is the nature of a woman to be jealous about her husband. This is a matter that all women feel whatever their level of piety and religion is. If any women could have been free from this, it would have been the mothers of the believers who were pure and whom Allaah thoroughly purified. Indeed 'Aa'ishah was jealous over the Prophet when he went out by night [as she thought that he left her to go to his other wives] so when he came back he told her: "O 'Aa'ishah, are you jealous?'' She replied: "How is it that a person like myself would not be jealous about a person like yourself?" [Muslim] However, if this jealousy is beyond the limits, it is dispraised and it might cause the breaking up of the family. So being jealous without a sound religious reason is dispraised and leads to bad assumption, the breaking up of families and other unlimited and unexpected problems. Allaah does not like this kind of jealousy. The Prophet said: "Allaah does not like suspicion without a sound religious reason [evidence]." [An-Nasaa'ee]

So, a woman should fear Allaah and she should not spoil her marital life by exaggerating in being jealous and embarrassing her husband with questions especially if he is a righteous man as the questioner mentioned. He is not to be accused as in principle a Muslim is innocent until proven guilty. Among the things that will help you to get rid of this dispraised jealousy is to supplicate Allaah as much as possible in order to remove this jealousy from you.  Besides, you should think good of your husband and stop questioning him, and stop being suspicious. You should get rid of these whispers and remember the danger of being suspicious about a Muslim as the Prophet said: "Beware of suspicion, for suspicion is the worst of false tales. And do not look for others' faults, or spy on them."  [Al-Bukhari]

For more benefit, please refer to Fatwa 89109. 

Allaah Knows best.