Aiui (and I am sure someone will correct me if I am wrong), someone who is a member of the Orthodox church may be a heretic in relation to Orthodoxy, but not in relation to Catholicism, and vice versa.
To be a heretic one has to be a full member of a church, and then decide to deliberately and knowingly deny one or more of its teachings. Even when the priest tries to convince such a person of error, they have to choose to remain in their wrong beliefs. Therefore, surely it is a nonsense to use the language of heresy in relation to full members of a different church; people who have never ascribed to a particular set of beliefs in the first place. Whatever the historical positions of those churches a thousand or more years ago, current Roman Catholics are not heretics irt Orthodoxy, nor vice versa. Martin Luther may well have been a heretic; that does not mean that every single modern Lutheran is in the same position.
I am not quite sure how any Christian calls another a heretic and then feels capable of adding, 'No offence'; nor how another can reply, 'None taken.' Surely the term is intended to offend. Therefore it seems to me that such language is really better avoided.
Ymmv.