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 (this Qur’an) in truth that you might judge between men by that which Allah has shown you (i.e. has taught you through Divine Inspiration), so be not a pleader for the treacherous (or deceitful person).
Few Reminders from the Above Aayah:
Do not argue for the one whose treachery [or deception] is known, such as the one who either pursues what does not belong to him or one who rejects a right he must fulfil, whether he is aware of that or not. In this is proof to [establish] that it is forbidden to argue based on falsehood and pleading [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]