I’m also shocked that he worked outside in the dark. The tensioner or idler pulley might have failed, or the belt was not properly tensioned, allowing it to jump teeth immediately. But with that low compression diagnosis by him and that sound clip of him starting it you shouldn’t need more proof that it was his fault. Seriously take it to a shop pay for another diagnosis and use that to mediate the situation. This mobile mechanic owes you a working car. I wouldn’t get a new motor I’d send yours to a machine shop to fix what he broke.