When my son was younger, his much older brother (18 years older) bought my son a video game. I can't remember what it was called, but my son knew right away it wasn't appropriate for him. He thanked his brother, but asked him if it would offend him if he traded it for something else. His brother had no problem with it. Not having any kids of his own, he didn't really think about his little brother's comfort levels.
Thankfully, my son, although young, knew he was not old enough for that game and made the decision himself to get a different one. My ex-husband and I didn't even have to use the "parent card" about it.
I guess there are a few ways you can handle this.
One would be to ask the parents if they can play something else when your son is over.
Two would be to let your son say that he doesn't like that game and suggest playing something else.
Three is to suggest that the boys play games at your house.
My son is 17. His repertoire has expanded as he's gotten older. However, he babysits for younger kids and won't allow them to play certain things when he's taking care of them. He certainly doesn't introduce things that are far above their ages with regards to what is appropriate.
I would hope that your son's friend would be happy playing other things with your son around. If his parents see nothing wrong with it, fine. He has all the permission and time to play those games. When it comes to your son, it would be good if they could play other things.
If he's not willing to do that for his friend, your son, then maybe they shouldn't play games at the other boy's house.
You're correct. You really have no right to tell other people what their kids can do in their own homes. If the friend gets a game, a video, a movie, or anything on the internet that your son knows isn't appropriate, he should just be able to call and go home. It might come to that.
Lots of people have allowed their kids to do things I never would have allowed my own children to do. They just knew to come home under those circumstances.
Again, there is always the option of the boys playing at your house so you can monitor and the other boy can do whatever he does at his own home.
Just my opinion.
Best wishes.