Oh OP,
So sorry to read this.
If it was me I would be so upset and pissed off at the same time.
However, you HAVE to keep reminding yourself that you did your best and HIS choices are why this has happened.
He was dishonest with you and insisted on this.
He made a bad decision and wouldn't be swayed from it, and he is paying the price.
You have to keep reminding yourself that he really needs to learn from this.
To do better.
To make better decisions.
To perhaps consider listening to others.
If he wants things to go better for him in the future, he needs to learn from this, or the same mistakes will be made.
I would be measured in my sympathy.
"Oh dear, that's a pity, hopefully things will improve for you etc"
If he rails at you, just remind him that he "wouldn't be told, no matter how hard I tried, you insisted on the flat, so try and make the best of YOUR decision".
It is very possible that he won't get as much out of the experience friendship/fun wise, but he may learn that decisions have consequences and if he had been honest with you and listened to you, he could have had a vastly different experience, which was all you wanted for him., ie that he would have the maximum fun possible.
I ran into a mother of a class mate of one of my sons and she was telling me how her son turned down an incredible opportunity in the States, over the summer, to teach sailing with a bunch of his friends, because of a relatively new girlfriend, who had to resit exams and wasn't going away.
This mum was so upset, however, there was no talking to him.
4 weeks into the summer she dumped him, and his parents had to look at his glum face the rest of the summer as he saw the incredible stories his pals put up on SM.
She said she vacillated between feeling so sorry for him and wanting to kill him for his stupidity and stubbornness.
He paid a big price for it and she made it clear at several points during the summer while she felt VERY sorry for him, it was through his own stupidity he missed out on the experience of a life time.
So sympathy yes, yet tinged with the positivity that he should make the best of the situation and learn from it if possible.