You can keep supporting the friendship while setting firmer limits on visits. I would pause sleepovers until you’ve spoken with the parents, and invite the friend to your house meanwhile.

**With your son, start with curiosity and take his answer seriously.** Being frightened by Harry Potter doesn’t prove he was frightened by this movie. He may have experienced it differently with a friend, or he may be reluctant to admit discomfort. You don’t need to settle that to establish your rule.

Choose a relaxed moment and say something like:

> “Thanks for telling us about the movie. You’re not in trouble for telling us. What did you think of it? Was there anything you didn’t understand or wanted to ask about?”

Listen without pressing him to describe disturbing scenes or admit fear. If he says he was fine, accept that and leave the door open: “Okay. If anything bothers you later, you can always tell us.” Frightening content can bother children later or cause nightmares, 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).

Then explain the boundary separately:

> “We don’t allow that kind of movie yet. That rule applies at friends’ houses too. Next time, you can say, ‘My parents don’t let me watch this—let’s do something else.’ You can also call us, and we’ll help.”

Don’t make his willingness to report what happened a reason to punish him. If the rule wasn’t clear before, make it clear now. Give him a practical way to reach you, but remember that supervising adults also need to uphold the boundary; a nine-year-old shouldn’t carry that responsibility alone.

**With the parents, be specific and calm.** First establish what happened, including whether they knew what the children were watching. Avoid opening with a judgment about their overall parenting.

> “Our son told us the boys watched *Slither*. We don’t allow him to watch R-rated movies or adult horror. We should have discussed movie rules before the sleepover. For future visits, could you check with us before showing anything beyond PG?”

Also address the bike and neighborhood rules directly: he needs his helmet, and you need an agreed area where he can play and a clear understanding of who is supervising. The AAP specifically recommends discussing supervision and media limits before visits in its [playdate guidance](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 understand and reliably respect your limits, start with shorter daytime visits. If they dismiss the rules or say they can’t enforce them, keep the friendship going at your home or through outings you supervise.

You don’t need them to adopt your parenting approach. You do need confidence that they’ll respect your boundaries while caring for your son.