There may not be a very clear answer to this question because women did not 'go out' in the old time. As far as I know and from the tradition that I am from, women mostly stay home and go out only for a valid reason. In most cases husbands has to consent with it or they will not go out.
There is a similar question here at Islam-QA.com. That answer references 33:33 which says
And stay quietly in your houses, and make not a dazzling display, like
that of the former Times of Ignorance; and establish regular Prayer,
and give regular Charity; and obey Allah and His Messenger. And Allah
only wishes to remove all abomination from you, ye members of the
Family, and to make you pure and spotless.(Yousaf Ali Translation)
This clearly advices women to stay at home. The answer says, if a women goes out they have to be cautious what they are going for is Halal and not forbidden in Islam.
One can also look at the life style of, Fatima (RA) who stayed at home and even was buried at night so that no one can see her.
This similar yahoo answer also explains what really are the rule for women when going out.
It is important to know that there are not too many hadiths and verses that talk about the issue. But if you look at the history, you will know that Muslims women stopped going to the mosque in the time of Umar (RA) second caliph. One can say if a women cannot go to mosque, why would she go out at night with friends. If she does go to the mosque, may be she is not the type of person who likes to go out.
In my she can go out at night (not with boys though nor in company of boys) only after the permission of a gaurdian who could be father or husband. This is pretty much established from what we know the ruling on Islam.
A side note, the prophet has forbidden going out after dusk as saying that shaitan comes out when the night falls. In that opinion, going out at night esp for women will be inappropriate. However I would suggest that you make up your own mind :)