Jill is an impulsive person. At that moment she sees MC as a victim of an assault. There is a certain fairytale logic to her actions. It is consistent with her naive sheltered princess stupidity, which she thankfully seems to be growing out of later on.
Why do people keep saying Jill is stupid? She's the less stupid of all the main girls... She may be "naive" in the strict sense of "inexperienced", but she's not stupid. If she were, she would be being sodomized by random guys across the campus, or fallen for Rusty or, even worse, Tybalt. Let's compare:
- Bella: dating a student, what could cost her job and, in some jurisdictions, her liberty (but nothing's gonna happen because we're living a fairytale).
- Maya: signing contracts without reading and she was kinda inexperienced too, right? She didn't even know she liked men...
- Sage: being a beard and being clueless about it...
- Josy: well... Josy's just Josy... She looks for hobbies on the Internet to fill her social media profiles...
The other girls? Well... Most of 'em suck cocks (or worse) through glory holes and/or are drug dealers and/or have crabs (because it's the only STD in our fantasy world, otherwise, it would be worse). It's not I don't like 'em (or the others above)... I probably wouldn't play the game without 'em... But it doesn't mean they are what I consider "standards of intelligence" or the kinda girl I'd marry in real life. Never been the marriage-type kinda guy but, if I were, the only one would be Jill. The rest would be just for fun.