This is harder for me to take a stance on than I thought it'd be. My only hope is that the parent knows what she's having her son play, and does more than the cursory glance over his shoulder during play. (Let's face it, all childhood gamers have developed parental radar and can tap a pause button as quick as a cobra snaps at a mouse.)
My initial reaction is that she's clueless, and just provides what he's asked for in their random conversations chock full of his "I want"s and "pretty please"s. But, my internal advocate pipes up and I have to try and give her the benefit of the doubt... at least until I piped up and asked her directly if she'd noticed the rating stamp, or seen any lengthy gameplay footage (and not just a preview... but a good 15min+ of gameplay and FMV time).
I can see both sides, though... My 4-year-old daughter's favorite TV Show for several weeks was 'Drake Game', otherwise known as "Uncharted 1&2". She was interested in the characters, was invested in the game in all the tense spots, and had a ball telling me "try again, daddy" whenever I had the screen go black and white. Now, she's all about Batman. From the pre-JLA seasons of "The Batman" cartoon (a great melange of classic Batman and updated character designs... with copious sprinklings of comic book references interspersed), to Nolan's "Batman Returns" and "Dark Knight". Heck, she's enthralled by Arkham Asylum when I get the itch to pop the disk back in the PS3. (except the Scarecrow sequences... those almost freak me out)
Then, there's my niece who is so empathic that when she sees pain, she practically feels it herself. She can't sit through a normal Disney animated movie, especially the classics. If there's creepy music and spooky lighting, she'll practically curl into a ball. If there are guns, or explosions, or anything even mildly considered frightening, she's not going to handle it... and she's 10. Her minor amount gaming time (~1-2hrs a day) is taken up with "Wizards 101", and her favorite TV program is Netflix streamed "Avatar: The Last Airbender" which I take as a sign that she's starting to mature past her mental blocks. (her halloween costume is a self-made Kioshi Warrior armor outfit).
I expect that when my daughter is my niece's age, she'll be able to handle SOME higher-rated games... but nothing before I've already put it through its paces. Perhaps not GTA or CoD/MoH, but certainly Prince of Persia titles and perhaps MGS or Assassin's Creed. Though, I expect she'll still have more fun watching, than having to learn how to play. But, she's already begun systematically killing off Sackboys by the dozen, so we'll see. (it's hard to hit more than one button at a time with tiny hands...)