The damage in relationships occur when the discussion turns from what is Catholic to who is Catholic–typically accompanied by strong implications that the person disagreeing with you is not really Catholic.
Of course, this is sometimes true, but when you are dealing with a fellow orthodox Catholic, making such claims is spectacularly unwise.
To use an analogy from training, we teach supervisors not to call an employee “lazy”–even if they might be–because, for starters, it’s pointlessly insulting and inflammatory. It’s counterproductive.
Instead, the supervisor should state objective facts, such as: “You have turned in your last 3 assignments late.” Now, the employee might be able to fix that! Focusing on timeliness gives the employee concrete, achievable goals. But if you just claim that he has a character flaw…that “lazy” is fundamental to his identity…then there is really nothing to be done about it: it’s too deep and nebulous. Rather, supervisors should approach it on the simple, less emotional, level.
Orthodox Catholics, by definition, follow the teaching authority of the Church: so why not just show them where they are wrong? Then they will change their minds! This problem arises precisely when we are unable to do so: when there is no authoritative Church document that is directly and unambiguously on point. That’s when the resort to personalizing the discussion often occurs.
When somebody sincerely seeks to put Christ, and His Church, in the very center of his life, and you then attack that center, hopefully there will be forgiveness…but it will never be forgotten that you attacked their very core. Trust may be never fully regained. I have seen this happen over and over again.
Moreover, it’s not our role as laity to determine whether a specific person is Catholic, adjudicate doctrinal disputes, nor declare somebody a heretic or schismatic. Refer the question to the bishop! But, bishops aren’t really into clarity and discipline so much these days: but many Catholics are willing to step into the breach…but without the authority to do so…and thus de facto overruling the very authority that they claim to be upholding.