Consider this, from Rev. 3:14-20
To the presiding spirit of the church in Laodicea*, write this:
“The Amen, the faithful Witness and true, the Source of God’s creation, has this to say:
I know your deeds; I know you are neither hot nor cold. How I wish you were one or the other–hot or cold! But because you are lukewarm, neither hot nor cold, I will spew you out of my mouth! You keep saying, “I am so rich and secure that I want for nothing.” Little do you realize how wretched you are, how pitable and poor, how blind and naked!..
Here I stand, knocking at the door. If anyone hears me calling and opens the door, I will enter his house and have supper with him, and he with me…”
*(Laodicea was a wealthy commercial center of the time.)
To answer your question, though, we cannot see into hearts. We don’t know that someone who is outwardly very observant, whether Catholic or Protestant, is inwardly in pursuit of God. We do not know whether someone who is outwardly lukewarm is getting more mileage out of meager graces than we get out of the bounty showered upon us.
For instance, perhaps they find Mass and their prayer life a great trial of dryness and distraction, while we find it a great consolation. Perhaps they have been wounded by their experience with the Church, while we have found only healing. Is it so much to our credit that we seek it out much more?
It absolutely matters where you are, of course. Nevertheless, a person who leaves the Church in spirit is better off seeking elsewhere than doing nothing, even if they do that nothing once a week for an hour in the pews of a Catholic church in the mistaken notion that just showing up in body makes them “rich and wanting nothing.” Ideally, they would stay and avail themselves of the riches of the Sacraments, but you are speaking of inherently non-ideal alternatives.
If they are devout and love Jesus, I hope you’ll have an opportunity to share with them about their faith journeys. You might make an agreement to listen to each other, to perhaps suggest additional things that each of you might consider doing, but not to attempt to act as if any of you have spiritual authority over the other. It could be quite profitable to all involved.