Context will dictate the definition although "aliens" is rarely used in BrE these days to refer to people from other countries. Many years ago, the signs at British airports to direct people to the correct immigration (passport control) desk, were marked "British" and "Aliens". Most people these days assume that the word "alien" refers to a life form from another planet.
Your second interpretation is not quite right. The sentence with "completely alien to her" is fine but it doesn't mean "strange". It means that she has never heard of these ideas before - they are unknown to her.
Student or Learner