I am writing a short story for an assignment and was wondering about something: would I write

"...she was a close friend of Ms. Harlan"
or would it be
"...she was a close friend of Ms. Harlan's"?
Thanks in advance...

If the person to whom you are referring is named Harlan, it is the first one.

If the person to whom you are referring is a friend of the one named Harlan, it is the second one; i.e. "..........she was a close friend of Ms. Harlan's friend"