wow, you're in left field.
we aren't talking about one specific person. This is a theoretical situation. I've known it to be an alcoholic or abuser or scumbag depending on the situation through my female friends. That is why i said that. I see it a lot and of course get introduced to this lowlife and it's like "WTF" or it's a person who you know because someone else knows him.
So basically the answer to your question would be, I know he's an alcoholic because I know who the person is. I'm not talking about just seeing some chick on the street with a scrawny guy and assuming the guy is a nothing. I'm talking about situations where you know the history of said boyfriend in question.