I didn't defend my friend

I have a best friend from preschool. We are both 16, 17 now. I absolutely adore him and everything about him. He ha autism and even though he's different I love him. I was having a birthday party sleepover and was inviting girls who also made cheerleading this year. I was excited and when I told my bestie I don't think he understood that I just wanted to have a sleepover with them. So he thought he was invited and came over and I didn't have the heart to turn him away. My friends were being pretty mean to him through out the night... At around 11pm they all really wanted to watch 'The Devil Made Me Do It'. But my best friend doesn't like scary movies and wanted to watch pirates of the Caribbean on Disney plus. They told him to stop being a cry baby. So we watched it and he got scared and they had it up really loud so the noises made him upset too and he went to the bathroom. He came out later and made a pallet for him to sleep on. So my best friend tends to wet his bed and especially after watching a scary movie, he wore his night time undies (diapers) and my friends could see it because his shirts were sagging and they started gull on bullying him. He cried and ended up walking home... I should have went after him. His mom called my mom the next morning and I'm grounded and those girls aren't allowed over. My mom is also making me write an apology letter for not being a good friend...... I feel so guilty...