He thinks she's making a terrible mistake, but she clearly knows how to use the gun (and that's disturbing in its own right, but he has issues with guns) and she is certainly not incapable of making decisions for herself.

He's also not overly familiar with 'kids' of any age group, and that combined with his own lack of experience being parented means that he's basically more inclined than most to treat them as adults. Especially when they're that close to being it.

So he isn't going to try and stop her. Just stand up there and watch her go. He'll jump in if there's trouble in sight, though.