Up until I was old enough to get them myself, I needed to convince my parents it was ok for me to play M games (Since I couldn't buy them myself), and that was an uphill battle at best. Most of the time, I didn't even bother as I knew the violence was going to make it a hard "no". Same with most of my friends in middle school.
And you know, it really didn't affect my enjoyment of video games. Sure, I played stuff at sleepovers at some houses, but that sort of "bad" behavior for a kid is always going to happen, and obviously, in smaller doses, playing M games and watching R movies is pretty benign. It's more about not normalizing excessive violence or other content for a kid when they're too young.
And even if that does happen, most kids are still fine, but I do think boundary setting is important and valuable between a parent and a child. Unfortunately, a lot of what you're describing OP may be total ignorance of video games on the part of the parents, and more broadly, ignorance of what the kid is up to.