<div id="friends-data" data-user-id="2" data-friend-id="8" data-is-following="true"></div>
<button name="button" type="submit" class="btn btn-follow followUser">Follow</button>

this.data = $('#friends-data').data();

<div id="friends-data" data-user-id="2" data-friend-id="8" data-is-following="true"></div>
<button name="button" type="submit" class="btn btn-follow followUser" data-user-id="2" data-friend-id="8" data-is-following="true">Follow</button>

$(document).ready(function(){
    $(".followUser").click(function(){
    window.alert($('#friends-data').data());
    window.alert(this.data());
  });
});