It is acceptable for an Orthodox Christian (or, for that matter, a member of the Polish National Catholic Church or a member of the Old Catholic Church - Utrecht Confession) to receive the Sacrament or Holy Mystery of the Eucharist in a Catholic Church. Grave necessity is not required. The position you are stating reflects the Church’s attitude pre-Vatican II, when administration of the Sacraments or Mysteries to an Orthodox Christian was permitted only in danger of death or the absolute unavailability of clergy of their own Church.
The corresponding Canon in the Code of the Latin Church:
Language to this effect can be found in the informational text included in most editions of the missalettes (?) used in Latin parishes. However, the text also indicates that Orthodox are advised to follow the disciplines of their Churches in regard to deciding whether or not they should do so. Presently, the Orthodox generally forbid their faithful from receiving the Mysteries or Sacraments from Catholic clergy. That doesn’t mean that there are not Orthodox Christians who do so and the Catholic clergy may not refuse to commune them under the provisions of Catholic Canon Law, Latin or Eastern, on the basis of their Orthodoxy.
Many years,
Neil