I might explain to your friend that there are cafeteria catholics just like there are members of other faiths protestant, orthodox or non-christian who don’t hold up to their own ideals, much less those held by their chosen institution.
I also wouldn’t let them come up with the excuse that there is christianity that is not “institutionalized”, all christians are all “institutionalized”, at the very least by the bible and Christ’s teachings. If this comes up, perhaps say something like “Don’t you know christians who reject Jesus’ teaching by their actions, don’t we all to a certain extent?”
If at this point you get him to listen to why we submit to an authority, then I would show (through the bible) how the authority of the church was established beginning with the OT and fulfilled in the NT.(Note to yourself, try to use examples. I don’t feel protestants are ready for the Catecism. I would only use it for your own knowlege and ability to articulate your position, unless you run across some of the many paragraphs that show the Catholic Church in a positive light as a teacher and upholder of the faith.
Something that has been swimming in my mind is the question, “How can a theology remain constant without the structure you see in the Catholic Church?” of course the answer is the Holy Spirit, but doesn’t this almost prove that the Spirit isn’t guiding protestant faiths, for they have not remained constant. In fact they have followed doctrines simply because they contradict Roman Catholicism…uh, don’t tell your friend this.
The Holy Spirit can give you better words than I, so dont forget to remember Him.
Good Luck and God Bless,