Answer
(Fatwa: 1217/1313/N=1433)
(1) If your friend admits that he himself or someone else on his behalf wrote the message and sent to that non-Muslim mistress mentioned there in the question and he confessed to give three talaqs to the wife in the message, then though your friend is liar in his acknowledgement, then too three talaqs took place on his wife as per the rule and she became haram for him. The same ruling shall apply when your friend attributes the message to someone else but in this case Shariah witness shall be required against him that he himself wrote the message or got it written by someone else and sent to that non-Muslim mistress.
(2) It is unlawful and forbidden in Shariah to give three talaqs to wife at once. Hence your friend should do taubah and istighfar. And if he has unlawful relation with that non-Muslim mistress and they even have child, then it is purely unlawful and haram as per the Shariah. Your friend should terminate the unlawful contacts and relations and do true and genuine taubah and istighfar to the Almighty Allah, otherwise he will have to bear huge loss in hereafter.
(3) Such person cannot be punished without Shariah evidence and the Shariah evidence is only attached with the Islamic Qazi. You need not worry about others, just advise him about taubah and istighfar if he has admitted before you regarding his unlawful relations; otherwise avoid thinking ill of others.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband