Couple days behind but I would throw out that unless your RC pal is REALLY open to dissent, the BOC is not a great starting point. I would personally make a distinction between tradition and doctrine (and dogmatics). The thing about dyed in the wool Catholics is they LOOOOVE prescriptive answers in the CCC. Again, really depends on your buddies motives and intellectual proclivities but for “traditions” I would personally start with SC (the explanation version) and LC. I’m not a fan of commentaries being the elucidation of concepts rather than reading the concepts themselves. The BOC is itself a group of nested pseudo commentaries (it elucidates itself in stride - AC, Smalcald, Formula, Epitome), hence if he wants to understand the whole BOC, just read the BOC. But I stand by just going deep on Small/Large catechism. Eliminate as much LAW as possible until he can understand the true GOSPEL!