Venerating Relics at a Non-Catholic Church

If a Catholic is visiting an Orthodox church that has relics of persons the Catholic Church recognizes as saints, they could venerate those relics, similar to how they can adore the Eucharistic Lord since the Orthodox have a valid Eucharist, which they reserve in their tabernacles.

Those opportunities should serve as a reminder of how much we have in common with our Orthodox brothers and sisters in Christ, and such occasions could be used to asks the saints whose relics are present to intercede that the Orthodox come into full communion with the Church. And one could also pray that Jesus, as the God-man, provide that blessing with his heavenly Father.

Regarding those who have broken with the Church more recently, be they sedevacantist (not recognizing that the current Pope is a valid Pope), formally schismatic or simply not in full communion with the Church while recognizing the Pope as validly elected, I would counsel not to visit those churches, as there will likely be more of a temptation to join those groups because they profess Catholicism to one extent or another.