If you read the book, you'll have higher expectations about the movie than if you didn't.
Personal Example) I read the Harry Potter books and watched every movie. I was slightly disappointed each and every time.
If your friend goes in expecting it to be exactly like the book, she/he are going to be disappointed.
If however, you watch the movie and view it as independent of the book, you'll enjoy it much more.
I read the Hunger Games series, I'm going to watch the movie, it's worth it.