Not sure where the 24hrs came from. But from my knowledge and what we did at work people are contagious for 48hrs after last poo/vomit. I guess that's why it's so easily spread because people think they are well and go back out into the world spreading it.
It can also live on surfaces for quite awhile so disinfecting regularly is important.

Hope they get better soon. But given it has lasted this long is probably take her to the doctor