If he's not supposed to ask, okay, he's not supposed to ask.
...it's a very simple spell, or at least, it looks that way from the effect; he's not enough of a magician that he can sense the detailed structure of her magic. So there could be a standard spellform for that simple effect, maybe as a piece of something larger, and the Summoned Hero could've run across it by accident while throwing magic around, and watched it snap into the shape-worn-into-reality, and then instantly acquired that Skill because Summoned Hero.
If she's just going around inventing entirely new spells with no worn patterns for them to snap into... actually he's not going to deal with trying to explain it. It'll be in the books.