"There is no need for "man" in that sentence. And normally, "lowlife" is a single word."
I agree, but to me the addition of "man" makes it sound very natural and colloquial. If one of my friends was inspired to say that to me, I would bet big that he would include the "man".
Great old Newyorker cartoon: Wealthy middle-aged couple at the dining table with their daughter and her boyfriend. Father says to daughter "Why does he keep calling your mother man?"