What should one do if they arrive late for the 'Isha prayer, and while they are reciting Al-Fatihah, another worshipper joins to pray with them; should they complete their recitation or restart Al-Fatihah, and what should they do if they are performing a supererogatory prayer?

If you are performing a fard (obligatory) prayer and someone comes to join you (as a follower) while you are reciting Surah (definition) Al-Fatiha, then complete its recitation and do not start it anew. Continue your prayer as you are, and do not change the posture of your prayer for the sake of a follower joining. This ruling applies to both obligatory and voluntary prayers.