Yup. Had this once before. When DS was 7.
I'd never been "not in your room please, stay in here" or constantly checking on him and friends that came over. They'd say they were off playing, and yes, his room usually would be in a mess, with cuddly toys on the floor, a few books on the floor, three games all set up and not put away.
Acceptable mess. Kids that have been playing mess. Mess that would take ten minutes to clear up mess.
Then he had one boy round, new boy that had just started at his school, they went off to play, he left, and DS didn't want me to go upstairs. So when he said, "Please don't look, you'll be mad" I knew something was up.
Everything in his room had been thrown on the floor. Some trampled on. Some of his toys had been urinated on. All the clothes were out of his wardrobe. To this day I've never seen anything like it.
The Boy had trapped DS in the corner and told him if he said anything he'd hurt him. DS was petrified, and watched this little prick destroy his room with a grin on his face.
I lost it and yelled at DS for not saying anything before he left, or coming to get me, which looking back I didn't handle well, because he obviously felt too intimidated too. Seeing the safe code "Mum, can we have some jelly" as "Mum, you need to come here, I need help in this situation" is a bloody excellent idea, truly. I'm sharing that one with my friends.
I phoned the mother, explained what had happened, I'd bloody want to know if my child had pissed allover someone's things, and her response was "Well he wasn't the only one in there was he." And hung up.
The next time I saw her in the playground it took all my strength not to urinate on her 
Your picture looks relatively similar, I'm so sorry you've experienced this. I now check on DS and mates too much, as a result.
Take a deep breath, get a bit of perspective because I know how mad you'll be feeling at this level of disrespect, no one got hurt, etc, so force a bit of calm, and know you'll never have those little swines anywhere other than under prime supervision, if you choose to let them come to your home again.