In The Name of Allaah, The Most Merciful, The Bestower of Mercy

Allaah [The Most High] said:

إِنَّآ أَنزَلۡنَآ إِلَيۡكَ ٱلۡكِتَـٰبَ بِٱلۡحَقِّ لِتَحۡكُمَ بَيۡنَ ٱلنَّاسِ بِمَآ أَرَٮٰكَ ٱللَّهُ‌ۚ وَلَا تَكُن لِّلۡخَآٮِٕنِينَ خَصِيمً۬ا

Surely, We have sent down to you [O Muhammad] the Book in truth that you might judge between people by that which Allah has shown you, so be not a pleader for the treacherous. [Soorah An-Nisaa Aayah 105]

Meaning: Do not argue for the one whose treachery is known, such as the one who pursues what does not belong to him or rejects a right he must fulfil, whether he is aware of it or not.  In this is proof to [establish] that it is forbidden to argue based on falsehood and to plead [on behalf] of one who makes a false claim –in religious or worldly disputes. On the other hand, it is permissible to plead on behalf of someone -in a dispute- who is not known for treachery [or deception].


[Source: Tayseer al-Kareem ar-Rahmaan Fee Tafseer Kalaam al-Mannaan. abridged & paraphrased]