The Gri-Gri will act as something of a “second-hand,” even if the worst should happen and the brake strand is released. Regardless of the context – sport, trad, multi-pitch, single pitch, trad, alpine – it is inherently safer than the ATC.

noun. gri·​gri. less common spelling of gris-gris. : a talisman, amulet, voodoo charm, spell, or incantation believed capable of warding off evil and bringing good luck to oneself or of bringing misfortune to another.

Daughter is using a Grigri as a self-belay device. This is expressly discouraged by Petzl, the manufacturer, on this page titled Self Belaying is Prohibited. That's pretty damning. As the page admits, some people do use a Grigri for self-belays, but it's not ideal and potentially dangerous.

If the person waiting in turn to rappel has their GRIGRI pre-installed on the rope, it can be inverted (and thus unblocked) by the weight of the person rappelling below them. So it will not be operational when the second person wants to start their rappel.