For one thing, you don’t know about the condition of the guy’s friend, who may have needed the guy’s assistance at times, necessitating their sitting together.
But I’ll tell you who is clearly in the wrong: your family, for staying and watching the movie, while your mother went home alone. If there was absolutely no way she could have stayed, you all should have left with her, and found something else to do. (Perhaps you could have gotten your money back too).
The OP said they had free preview tickets (so no refunds), and the Mum would have been more annoyed if they’d all (a big family party) foregone them than if they’d stayed and she had to go, which is understandable.