Was it the only FA? I believe FA's often dead-head (right terminology?) on flights they aren't working and still sit in the jump seat. If this isn't the case, then yes, very poor form.

I was on a DEN-BOI flight last month where I was appalled that one of the FA's just sat and read "People" in the jump seat the entire flight then I remembered watching the same FA walk on late and introduce himself to the other FA's and I finally realized it was probably an FA catching a ride home or repositioning as there were enough other FA's.