People get so emotional over court filings. He's not saying any of this with his filings. Essentially, he's saying that at least some part of her alleged injuries ("severe, traumatic and permanent injuries and mental anguish") are attributable to the parent's actions/inactions, and that in order for the claims to be completely satisfied, the parents must be joined as defendants. However, his angle in this is not to protect her interests, but to protect his own. Assuming his allegations are true, arguendo, then the parents would be in some part responsible for the injuries alleged in the complaint ("severe, traumatic and permanent injuries and mental anguish") and the amount of money he would owe would be reduced.
To use a non-pedophile example, say you're hit by a car driven by John Smith on Monday and you break your arm, and then on Tuesday you're hit by a car driven by Suzie Brown and you break your leg. If you sue only Suzie Brown for your injuries, she can bring John Smith into the litigation as he bears responsibility for part of your injuries as well, and (look at the rule below) not having John Smith be party to the lawsuit means that you would "leave an existing party (Suzie Brown) subject to a substantial risk of incurring double, multiple, or otherwise inconsistent obligations (i.e., paying for the medical bills related to your broken arm) because of [not also suing John Smith]."
Fogle's saying that, assuming a jury finds he caused the plaintiff "severe, traumatic and permanent injuries and mental anguish," he wasn't the only one who did so, and that her parents contributed to part of those injuries and should therefore be held legally and, more importantly, financially responsible for compensating her for those injuries.
This hasn't been a commentary on the merits of this filing, just trying to explain what's going on from a legal perspective.
Here's the relevant federal rule (FRCP 19):
