No, she definitely thinks the Catholic Church holds the fullness of truth vs the other Christian denominations. We’ve had a couple incredibly beautiful conversations about that recently. And she went to daily Mass with me yesterday and today! It’s more of “why doesn’t another day of the week suffice for the Mass obligation and I can go with my boyfriend on Sunday to his church”.
I might go at it another way.
When it comes to obligations we have under God, there will almost
always be a moment when we have to choose to obey before understanding why to obey. Think of Eve: She knew God had told her not to eat of the fruit of the tree in the middle of the garden (just one specific tree to avoid! She could eat from all the others!) but somehow all the other trees weren’t enough for her, she wanted to have that one too. She didn’t necessarily seek it out… but when the serpent came to tempt her, she had a choice: obey God and decline that fruit (even though the serpent made arguments that appealed to her mind and desires!) or disobey God and eat that fruit.
It might’ve seemed arbitrary to Eve, but it wasn’t arbitrary. There may well have been other reasons (presumably all in accord with Eve’s best interests) that God told Eve not to eat that specific fruit… but even beyond them, no matter what they were, Eve sinned by not trusting that God loved her and had her best interests at heart when He gave her His commands. She should’ve chosen to follow God’s instruction just because He said so, even despite the subtle and clever lies the devil was telling her. She should have clung to the simple instruction of God. Obey first, ask God more about it later.
Hopefully the analogy makes sense here. Please forgive me if it doesn’t.
Beyond that, I might suggest considering together… why does her boyfriend’s church celebrate on Sunday? Why can’t they visit his church on a different day of the week? (I’m assuming it will be because his Church only
has services on Sunday). Really explore what it says about the importance of Sunday as a day of Christian worship. Her very boyfriend’s own church’s practice of holding services on Sunday, offers a hint back to the root of why Catholics hold this to be an important day of worship too (so important that it’s the day of official obligation in the Church, whereas Masses on other days are attended but optional).