Q:

Is meher payable even after a divorce demanded by wife? 

Sometimes the meher is not given, or is not given in full, at the time of marriage, rather the man promises to provide the meher at some later date. 
This question regards the situation where the wife demands a divorce from her husband, and the husband decides to let her go: Is providing the complete meher still compulsory for the man? 

A:

According to Jafari Fiqh Mahr is her right, to which her husband remains indebted unless woman forgive it (for example in case of Khul divorce or Mubarat divorce)
And should be paid in any situation and even can be asked to be paid legally by sue.  Unless the woman forgive it willingly.  But this may depend on laws of each country.  For example in Iran the amount of meher is registered in marriage document as a legal document and the woman can sue husband for not paying it before or after the divorce.  It is like any other legal debt. 
Islam recommends to meher be paid as soon as possible even if no divorce happen as well as any other debt.  There are some Hadith saying remaining under debt is Makrook (disliked by Islam). 
Ref:
http://www.al-islam.org/marriage-handbook/6.htm#6
Quran says:

وَآتُوا النِّسَاءَ صَدُقَاتِهِنَّ نِحْلَةً ۚ فَإِن طِبْنَ لَكُمْ عَن
  شَيْءٍ مِّنْهُ نَفْسًا فَكُلُوهُ هَنِيئًا مَّرِيئًا
[4:4] Sahih International
And give the women [upon marriage] their [bridal] gifts graciously. 
  But if they give up willingly to you anything of it, then take it in
  satisfaction and ease.