Let me tell you a brief story....
I planned a Halloween themed sleep over for my son and his friends. It was the weekend before Halloween and I'm the mom who always took them trick-or-treating, but Halloween was on a school night.
I let the moms know that we would be having dinner, making popcorn. and watching old scary movies.
One mom FLIPPED!!!!!!
I think the boys were about 10-11. She said her son was NOT allowed to watch anything scary because it would give him bad dreams and traumatize him for life...............
I went to her house and let her watch the movies. She said, "THIS is what you were talking about?"
There was no Chainsaw Massacre, no Excorsist, no Jason and no Freddy Kruger. These movies were so old and so bad that they were hilarious. There was nothing graphic, mostly lots of dark and fog machines. One minute a man is sitting in front of the fire in a big house and the next minute, he's walking in the foggy woods for no apparent reason.
The mom gave her full permission once she saw what type of movies I was talking about.
If you have perameters, then I would call her and talk to her about it.
Don't tell her what she can and can't do in her own home, but ask her about what activities she will allow the kids to engage in. Let her know your son may not be comfortable with some of it, but give her a chance to explain and show you what they might be engaging in.
For my Halloween sleep over, I had little boys in sleeping bags on my living room floor. I had my sleeping bag on the couch. I stayed with them until they fell asleep. They didn't do any video games or anything because that wasn't on the party menu. Period. They still had a great time.
Talk to the mom. If you're truly worried about your child being uncomfortable, he doesn't have to go, but she may be willing to make some concessions. You never know.
Call her.
Best wishes.