You can set a firm boundary about movies and keep supporting the friendship. I would pause sleepovers and visits at their house until you’ve spoken with the parents; invite the friend to yours meanwhile.

With your son, start by accepting his account of how he feels. Being frightened by *Harry Potter* doesn’t establish that this movie frightened him. A calm conversation gives him room to tell you more:

> “Thanks for telling us what you watched. What did you think of it? Was there anything that bothered you or that you didn’t understand?”

Listen, and answer whatever he actually asks. Avoid repeatedly asking whether he was scared or telling him he must have been. Then explain your boundary:

> “We don’t allow that movie at your age. You’re not in trouble for telling us. We’ll talk with your friend’s parents about what you can watch when you visit.”

Give him a practical way to handle another uncomfortable situation: “You can say, ‘My parents don’t let me watch this,’ suggest something else, or call us. We’ll come get you whenever you need us.” Make sure he knows how to reach you, and let the hosting parents know about that arrangement.

Over the next few days, notice how he sleeps and behaves. If he brings up frightening images, acknowledge the feeling, explain that the scene was created with actors and effects, and offer comfort. Frightening media can bother children days afterward, according to the [American Academy of Pediatrics](https://www.healthychildren.org/English/tips-tools/ask-the-pediatrician/Pages/should-i-let-my-child-watch-scary-movies.aspx). If distress persists or interferes with everyday life, contact his pediatrician.

Talk to the parents privately, after you’ve had time to settle your anger. Establish what happened and make your expectations specific:

> “He told us the boys watched *Slither*. We’re concerned because we don’t allow him to watch R-rated movies. We’re glad they’ve become friends, and we’d like to agree on what happens during visits. Please check with us before showing anything beyond [your chosen limit]. Can you make sure that happens?”

Discuss the outdoor rules too: he needs his helmet, agreed boundaries for where he can go, and the supervision you expect. The AAP specifically recommends discussing supervision and media rules [before playdates](https://www.healthychildren.org/English/safety-prevention/at-play/Pages/Questions-to-Ask-Before-a-Playdate.aspx).

Their response should guide your decision. If they take your concerns seriously and can reliably follow those rules, you could try a short daytime visit. If they dismiss the rules or say they cannot enforce them, keep visits at your home. Explain that arrangement to your son calmly, and continue welcoming his friend.