Interesting question, I believe its a heart thing. I have heard several accounts of similar things one being an Amish man who came to the Lord and began to see past all of the Amish rules. He began to follow the Lord and guess where the Lord lead him? Right back into the Amish Church this time not to follow all of their rules and doctrinces of dress but to be a witness. He does however out of respect and a more powerful witness do many of the things the Amish do but NOW with no true religious conviction but rather that he may win some to the Lord. He does however break the some of the rules like Reading the Bible in plain English, the Amish believe you should read the bible in Old German, the language of their ancestors.
Also doesn't our Dear brother Paul say He was a Jew for the Jews but not under the Law? He became all things for all men that they maybe saved. What would be the difference here? A disconnect of the Heart. If their are 100% trusting in Christ for their salvation and only "doing the Catholic things" to stay as a true witness of Christ Jesus then I wouldn't see anything wrong with it. Now I believe it would be wise to stop with some of the heretical things of the ROMAN Catholic Church like Popish worship, praying to Mary/ the Saints, etc.
God sees the heart more intently than any and knows if their person is doing this for the correct reasons.
I am another example I suppose, I use to have terrible language along with the friends that follow this, did I just abandon those friends? No, why? Because my heart became further from the sin that it use to love such as swearing, which they still do but know that its no longer apart of my vocabulary and so use it far less frequently around me because I have now become a witness to them that "Hey, maybe cursing is wrong." Also know I have met many people that have cursed and cussed for so long they no longer view it as a negative but with they feel or hear the contrast of my new vocabulary it causes them to really look at how they talk. BUT again its all about the Heart and its all about being a Witness of Christ Jesus and how he can transform your heart into something completely new.
I already know many of you will probably disagree with me on this one but thats okay, We are all growing at different rates and speeds and this answer is currently where I am at, I do hope you show some grace. :)
*Edited Additions added later* Also I have seen what I would view as a system of Pharisism in the average local church. The do's and don'ts of Christianity, like going to church religiously to worship God in an hour meeting time, looking at the back of the head of some guy ahead of you in the pew. I look at the bible and see something different and alive, BUT I still go to church, but now its out of Love for my Brothers who are still in bondage to religion clinging on to the hope that maybe if they go to church enough they maybe saved. Wouldn't it make sense that this form of religion is just as wrong as the Roman Catholic church? I stay as a witness to them that its ALL CHRIST. I disagree with the "format" and how church is done on the modern whole but because my heart knows the truth and my heart is disconnected I am uneffected by the Religisoity know as "modern Churchianity" but my heart is ever connect to those that need them and it even breaks for them as I see them still feel empty as I once did.
All in all whether its in the form of religion or the form of swearing, the Flesh is the Flesh and it truly STINKS like the dying corps it is. If God has told the man to stay then so be it as long as His heart is connected to the Father and disconnected to the Roman Catholic church.*
God Bless,
Matthew