I fully agree with you on this point. There is only one , chided you in his remarks. Insulting another Muslim because he is from a different sect is unjust and unfair.
I could give him hundreds of argument to prove him wrong. For example he had said that Hazrat Fatima died in young age. Shia believe that she was attacked by Hazrat Umer. My question to him is , if that is the case then why her husband Hazrat Ali who had a title of Sher-e-khuda did not protect his wife. In fact he did nothing. How can it be possible for any husband to see his wife being murdered and he stay quiet and watch. Let alone someone as brave as Hazrat Ali. I am not expecting him to convert to be a Sunni. My point is that one should respect other sects otherwise there will be people like me who will pose a question to which they will not have an answer. Let's see if there is only one has an answer to my question.