السلام عليكم احسن الله إليكم What is the correct way on acting upon what was mentioned in tirmidhi 220? E.g A friend needed someone to pray isha with him after main jama'ah, so I joined him and offered 4 units but as nafl. What is the ruling on doing this during prohibited times of salah?

Yes, you're correct. You can join him. If he is the Imam and you join him, then you follow him in prayer. If you are the Imam, you pray two rak'ahs and he gets up and finishes the rest. Following an Imam who is praying a voluntary prayer while you are praying an obligatory prayer is correct and valid.