There are two paths for britney, one where the player is skinny and one where they are fat. In both cases continue to party or visit her until you reach the end. I think there is a clear ending for the skinny player path but not for the fat player path. So if you are a fat player it will sort of just loop once you reach the current end, though it unlocks a “side” route that naturally occurs as you play until reaching an official “ending”. So if you get the official as a player you have completed all of the fat player+britney content