Islaam has also prohibited a man from being in privacy with a woman – which means that he is alone with her in an isolated area with no one else present – since this leads to the occurrence of fitnah.
The Prophet صلى الله عليه وسلم said: “Beware of entering in the presence of women.” The Companions said: “O Messenger of Allaah, what about the male in-laws (i.e. the husband’s male relatives)?” He صلى الله عليه وسلم replied: “The male in-laws are death.”  This means: The danger of these family members is greater. Why is this? It’s because a woman is less restrained from her husband’s male relatives as compared to other males. Her refraining from them is minimal. But in spite of this, there must be caution and awareness applied to a husband’s male relatives.
As for what we hear about today from some ignoramuses that a husband’s brother, paternal uncle or other male relative can greet his wife, shake her hand, be alone with her and enter into her presence – this is baseless. It is not permissible for a nonmahram to enter into the presence of a woman (without Hijaab), nor to shake her hand, nor to be alone in privacy with her unless there is someone else in the house through which the privacy will be removed. But as for him entering into a house that has no one in it but her, and he is not one of her mahaarim, then this is the forbidden type of privacy and it is dangerous.
Another example of this is if he enters an empty room – there being no one in there but him and her. This is not permissible since it is a means that leads to fitnah. This is even the case if the man that is alone with her in this private area is a doctor. The Prophet صلى الله عليه وسلم said: “No man is alone in privacy with a woman except that the Devil is the third party amongst them.”  This means that the Devil is present and makes falling into immorality appear pleasant to them. This is since he is always calling to fitnah and takes advantage of this opportunity to spread immorality between them. In order to cut off the pathway of the Devil and his supporters as well as the path to corruption, the Legislator has restricted men from being alone with women.
From the new forms of this forbidden type of privacy that has appeared in our times is a woman riding in a car by herself with a driver that is not mahram to her. So he takes her to the school, to the market and even to the masjid! This is not permissible. It is not permissible for a woman to ride in her car alone with a driver that is not a mahram to her because this is from the prohibited forms of privacy.
 Reported by Imaam Al-Bukhaaree (2/219) from the narration of Ibn ‘Abbaas radhi Allaahu anhu
 Reported by Imaam Al-Bukhaaree (6/158-159 from the narration of ‘Uqbah bin ‘Aamir radhi Allaahu anhu
 Reported by At-Tirmidhee (no. 1171) and a similar narration by Imaam Ahmad in his Musnad (1/18) both from the narration of ‘Umar bin Al-Khattaab radhi Allaahu anhu
Source : Advice to the Muslim Woman – Shaykh Saalih bin Fawzaan al-Fawzaan [al Ibaanah Book]