What is the best way to deal with a righteous friend who is, however, bigoted towards sheikhs who are not from Ahl al-Sunnah? My mother does not understand Arabic, and he delays Maghrib prayer in Ramadan.

It appears that your friend's shaykh is an innovator (mubtadi') and not from among Ahl al-Sunnah wal-Jama'ah (the People of the Sunnah and the Community), due to his preference for Ali ibn Abi Talib over Abu Bakr and Umar, may Allah be pleased with them both. This contradicts the consensus of the scholars of the Sunnah. Furthermore, his considering it a great error for a traveler to complete his prayer behind a resident is a significant mistake. Wearing a necktie is not considered imitating the disbelievers nowadays. As for the Prophet, peace and blessings be upon him, breaking his fast completely before Maghrib prayer, we do not know of that from him; rather, what is reported is hastening the breaking of the fast with fresh dates, dried dates, or water. The shaykh's declaring as misguided anyone who disagrees with him among the scholars is an indication of his bigotry and ignorance.