When it comes to the way in which the terms are actually used in practice by the Orthodox, I would agree with your interlocutor that “heterodoxy” is less grave than “heresy”, though this is a little paradoxical inasmuch as the former word betokens a larger, and more comprehensive, difference of viewpoints. Thus most Orthodox would regard Roman Catholicism and Protestantism as “heterodox”, for each comprises an ensemble of differences making them truly different darshanas (the Greek doxa being tantamount to the Sanskrit darshana), though at the same time of course each of these other Christian communions shares certain fundamental dogmas with the Orthodox. By contrast Arianism is a “heresy”, even though it is based on a less encompassing difference, for it willfully denies an explicitly formulated doctrine of the Church, one obviously of the highest magnitude.
There are doubtless other nuances that one would have to consider, but it seems to me that there is a certain correspondence between heterodoxy and what Schuon calls “extrinsic heresy” and between heresy as such and “intrinsic heresy”.
So, if we consider “heresy” as having the above double meaning, as Orthodox Christians what our stance should be against groups such as the Oriental or Anti chalchedonean Christians? Of course their doctrine is by no means the same in quality with that of Arius or Eutychis, but they have been condemned for their techings in Ecumenical Councils. So here is the big question: How we can admit their orthodoxy in a manner which not renounce the validity of the decisions of Ecumenical Councils?