That’s not what the canon law says. The Latin code says that, but that code doesn’t apply here. The Eastern Code says that an Orthodox Christian (such as a Russian Orthodox, from the original post) can indeed be a Godparent along with a Catholic Godparent–and that code is the one which applies because the parents are Eastern Catholic.
See the Eastern code, canon 685, which has been quoted here a few times,
not the Western code, canon 872–also quoted but not applicable.
I will take your word for it.
But I found this, Canon 685, 1990 Code of Canon Law on the Oriental (eastern) Churches:
Canon 685 - §1. For a person to fulfill validly the role of a sponsor it is necessary that he or she:
1° be initiated with the three sacraments of baptism, chrismation with holy myron and the Eucharist;
2° belong to the Catholic Church, with due regard for 3;
3° have the intention of carrying out the responsibility of sponsor;
4° be designated by the person to be baptized or the parents or guardians, or, if there are not any, by the minister;
5° not be a father, mother or spouse of the person to be baptized;
6° not be bound by excommunication, even a minor one, suspension, deposition or deprived of the right of acting in the function of a sponsor.
§2. To assume licitly the role of sponsor, in addition to what is required, the sponsor should be of the age required by particular law and lead a life in harmony with the faith and the role to be undertaken.
§3. For a just cause, it is permitted to admit the Christian faithful of another Eastern non-Catholic Church to the function of a sponsor, but always at the same time with a Catholic sponsor.
So, you are correct. But I doubt that a Catholic can be a sponsor in an eastern orthodox church. Is this correct?