Can Catholic Who Joined Orthodox Church Be Saved?

That depends on whether they die in a state of grace or not. Leaving the Catholic Church is certainly grave matter, but, to be subjectively guilty of mortal sin, a person needs to have committed grave wrongdoing with full knowledge and complete consent (CCC 1857-59).

And to not be saved, a person needs to persevere in that state of mortal sin until death, which may not be completely manifest when they die. Ultimately, God judges souls. And we pray for their reconciliation in the interim. See also CC 846-48.