If on the part of the Christian in question, an absolute refusal to submit to the Roman Pontiff, but yet still presents himself for communion, then he is a schismatic and cannot commune. However, as this is not the case with many in the various Eastern churches, and since by taking communion in the Catholic Church, they acknowledge the legitimacy of, i.e., the Filioque, then they can do so.
The Orthodox churches prohibits the reception of the sacraments from non-Orthodox ministers except in an emergency, and the Catholic Church prohibits Catholics from receiving sacraments from non-Catholic ministers except in the case of an emergency, and so the same rules follow in this regard, so, while a good-willed Orthodox Christian may be communed in the Catholic Church per Catholic canon law, per Orthodox canon law, he cannot. However, it would be a sin against conscience to return to receiving communion in the Orthodox church after doing so in the Catholic Church, since he would again separate himself from the Catholic Church and her faith. Neither Catholic canon law, nor Orthodox canon law permit Catholics to receive communion from Orthodox ministers outside of an emergency.
Benedicat Deus,
Latinitas