Here is my question: I generally know how to use the apostrophe for possessive tense; however, the “s” has ALWAYS confused me. So a friend of mine is writing a paper and I'm proofreading it for him, but I'm not sure I am correct. He's writing an essay about a book by Frederick Douglass. 

So when he is writing the possessive for Frederick Douglass, should he be putting the apostrophe after the second “s” and leaving off the third “s”, or would he still add the third “s”

Should it look like: Frederick Douglass'

or should it look like: Frederick Douglass's