How about this case? It's a true story that happened recently.....
A traditional Catholic husband and wife (14 kids) who practiced the traditional faith for the last 50 years or so, apparently lost their ever loving minds and decided to "convert" to the Russian Orthodox church. Before RO priests and prelates, a ceremony was held where they took solemn vows renouncing their baptism and everything Catholic that disagreed with RO, then they were baptized and welcomed into the RO.
It was with only the best of intentions that these poor fools left the Church. They believe they've left the Church because that's what they did. They believe that they are now RO so they are no longer Catholic. Believing that they finally found the truth, they decided to purposely and certainly place themselves outside of the Church.
Now, while Holy Mother the Church teaches us to avoid these Apostate, schismatic heretics, *She* calls on them to return to the Church, that is, She calls on them to get to confession and amend their lives.
The process for this is so simple that all they need to do is to enter the confessional, just the same as they did regularly for the past 50 years. Not only *can* they do this, they must do it for their hope of salvation. This simple, beautiful option which can only be partaken of by Catholics, remains open to these Apostate, schismatic heretics for as long as they live because what they did was commit mortal sin - even though they had good intentions.
The nature of the sin they committed makes it unlikely that they will ever make use of the sacrament of penance and return to a life of faith within the Church, but as long as they live (which being in their upper 70s may not be much longer) there's hope because with God, all things are possible. Please pray for them.