Know all of what? That it’s not always easy for someone to completely leave? If it were, there would be even more Catholics who might not be as faithful as someone else, leaving for another place to worship. But instead they maintain some ties. It has to be hard for them to try holding onto parts of the faith, if it’s even only an identification based on Baptism, when they are faced with people telling them to get out. It has to be hard for them when they are told by the Church they are Catholic, but as hard as some might try to form their conscience to Church teaching, their conscience at the point of their journey they are at, can not abide by everything.
The Church does call her a Catholic by virtue of her Baptism so that’s how I know that.
Catholics from a very young age with even the very basic instruction are told the Catholic Church is the one true Church.
I did say “maybe” she fears if it is the one true, that she then remains. I obviously haven’t spoken to her to say for sure. That’s why I said “maybe”.
And it should be obvious to anybody that one never knows when another might repent and return to full communion. Only God knows the heart and the future. So that’s how I know that.