An Apostrophe Query

I know this is a grammar board, but I figured this would be the place to inquire about the apostrophe. In a recent newspaper article in a local publication, the last name of a gentleman was Williams. Because his name ends in an "s" and it's a proper noun, shouldn't the possessive be "Williams's" and not "Williams'"? A friend tried to tell me that it's acceptable to use just the apostrophe because it ends in "s", but I argue that because it's a name, it needs apostrophe and "s" both. Who's correct?