This from the woman who jumps on everyone for ignorance and racism while basically saying to the OP 'you're not white, are you?" So what iif he isn't. If the man is a US citizen then he's American, that's a fact. Doesn't matter what race or colour he is, where he was born or where his parents were born. He's a US citizen, so for the purposes of foreign travel, he's American.
how difficult would it be to get a visa?
Impossible to guess. It's ostensibly because the US hasn't yet honoured its promise to give visa free travel to a few accession states but to be honest it's probably security related and designed to stop terrorists coming into Europe on 'holidays', so the conditions could be quite stringent. But if you're an honest, regular US citizen you wouldn't have to worry. Sometimes in life you just have to grit your teeth and deal with bureaucracy.
They even said that there were times that they had to talk to an immigration lawyer to stay longer.
Well yes, obviously you're not allowed to outstay your visa. If you want to go for longer than a standard holiday visa you would need a working visa which is another matter entirely.