Even when someone you know is on the side of err, remind them with love and compassion. Do not help shaitan against him. Although at times his ignorance offends you, and although he is just terribly mean, do not incite others to hate him by exposing his faults and affairs. Either by naming him and basically exposing him, or describing his etiquette which is well known to people (or not) without naming him. After all, the end result is the same thing. No matter how small a defect you happened to reveal about him, you've backbit him.

"Those who love (to see) scandal published broadcast among the believers, will have a grievous penalty in this life and in the Hereafter: Allah knows, and you know not." { Quran 24:19 }

Dont let his niggardliness develop hatred in your heart. Show him mercy when he speaks about matters that which he doesn't understand. Take him as a child who needs help to stand on their toes. Embrace him when he asks you a question. Do not fret about being disturbed by his curiosity. Answer him without virtually shouting out 'you are wrong!'. Allah is probably testing your patience through him. Face him with a clean heart, and a smile that is skin deep.
Mean it when you say, "I am here to help". Exercise your thoughts, make yourself feel rewarded to be able to rectify the affairs of whoever you know. Show mercy to receive mercy in return.

عَنْ جَرِيرِ بْنِ عَبْدِ اللَّهِ قَالَ قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا يَرْحَمُ اللَّهُ مَنْ لَا يَرْحَمُ النَّاسَ

Jareer ibn Abdullah reported: The Messenger of Allah, peace and blessings be upon him, said, “Allah will not be merciful to those who are not merciful to the people.”

(Sahih Bukhari 6941, Sahih Muslim 2319)