Our Lord Forbade Us from Pleading On Behalf of Those Known for Treachery In Religious Or Worldly Affairs
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]