It's because it's Phil, a person with such an extensive history of deviousness that makes it VERY difficult to believe that he didn't deliberately target a minor to con into being his new mommy (which carries such disturbing implications that I don't want to get into). We would be saying much the same thing if she had been 20, that he's a gross person for what he did, and it wouldn't really be better, but there's no denying that it says things about Phil, the fact that his first girlfriend was 17 when he first started chatting her up. While he was 30 years-old.
That being said, it's at least somewhat a saving grace that Phil wasn't interested in fucking her, but in having her clean up after him forever, which is why I don't consider it "grooming" in the sense that term is typically used