In fact it is not hard to add a "Facebook like" to his site

  • Register as a Facebook Developer
  • Add your site as an application site
  • add the javascript+html script piece that goes well by updating the application identifier 'xxxxx'

For example to add a like insert the following code:

 

facebook.js file

 window.fbAsyncInit = function() {
    FB.init({
      appId      : 'xxxxx',
      xfbml      : true,
      version    : 'v2.6'
    });
  };

  (function(d, s, id){
     var js, fjs = d.getElementsByTagName(s)[0];
     if (d.getElementById(id)) {return;}
     js = d.createElement(s); js.id = id;
     js.src = "//connect.facebook.net/fr_FR/sdk.js";
     fjs.parentNode.insertBefore(js, fjs);
   }(document, 'script', 'facebook-jssdk'));
   

Article

I have included both parts (html code + javascript ) in a plugin