They are supporting a schismatic church. Like I said, if they were taught through generations that their church is the correct one and is the one doing what Christ wants us to do, etc. then it’s a different situation from someone who is a Catholic and walks away. The person who is taught for generations that their non-Catholic way is the correct way may be less culpable.
However, the Catechism is very clear that “Outside the Church there is no salvation”, which means that these people are not achieving salvation directly through their schismatic churches, but only through the fact that their schismatic church may be in communion with the Church in some important respects (for example, Trinitarian baptism, and in the case of Orthodox, valid sacraments (though they are illicit in the eyes of the Catholic Church)).
Additionally, if the person has reasonable access to a Catholic church and rejects it in favor of continuing to attend the non-Catholic church, then to some degree he is culpable for that in God’s eyes. Maybe “in schism” is the wrong word to use for the individual, but they are definitely like I said attending and supporting a schismatic church. This is not okay. Even if you make some argument that it’s 90 percent okay or whatever, it’s still 10 percent not okay.
By continuing to attend and support a schismatic church, the attendees propagate the original schism and its errors. If they all converted to Catholicism, the schismatic church would cease to exist, which would be a desirable outcome.