And he is an Orthodox (just that) as καθολική χριστιανή in the other example (the one of the friend from Italy) only accepts Catholic and not Christian Catholic? I know there are differences within Orthodoxy as there are in Catholicism, but it can be understood as a general statement... (I think)
"Orthodox" in English is very rarely if ever used as a singular noun. However, it's fine as a plural/collective noun ("The Orthodox believe in...") or an adjective.
Why is "Catholic Christian" wrong in the other question but "Orthodox Christian" the only accepted translation here? Is Greek really as schizophrenic as all this?
I'm afraid I can't say I see your point here.
Firstly, what was the other question? You'll have to be a bit more specific than that.
Secondly, I'll guess what you're saying is that, Catholic was an accepted translation in the other sentence, all by itself while here, Christian is needed. I guess that's because, as you can see here https://en.wikipedia.org/wiki/Orthodox , this term is being used for other religions as well.