Nah, your friend is worrying over nothing, been watching too many movies maybe. Sure, how would anybody know if he’s catholic, as long as he’s not wearing his “I love the Pope” t-shirt? Look, you’ll see a lot of weirdo loyalists and an overabundance of union jacks in some places, but they’ve no interest in you. Likewise, catholic areas will have lots of interesting murals celebrating various anti-colonial struggles, you won’t find much overt sectarianism in those neighborhoods. Leave the Pope t-shirt at home (I guess there’s no presbyterian equivalent?) and avoid the dodgy parts of town, you’ll have a great time. The Antrim coast and Mourne mountains are very beautiful, if you like a bit of nature!