Your bully lied and told your girlfriend you cheated on her. She was so angry, she didn't even bother to check if it was true. She went out with "friends" and sent you this picture saying "Hope she was worth it"
Its not that you hated your BFF or wanted them to suffer - but your best friend's sister was so cute and well so was his GF - could you really pass up the chance with both of them? Maybe your best friend will never even know you used their sister and GF