Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with this situation today.
Unfortunately, if these cats all have green discharge, this tells us that we have a bacterial infection (+/- an underlying viral one) and antibiotics will be indicated. Therefore, despite the new owner's wishes, your neighbor is right about this situation. They need to be seen and treatment dispensed to clear this for them. And while flu is not often fatal, there is serious risk that the eye infection could permanetely damage their eyes. So, much so that it is not uncommon for untreated infections of this nature to lead to ulceration and/or even perforation of the eye (which would then require surgical removal of the remnants of the eye). Furthermore, if they are not eating/drinking properly, we will see weight loss, weakness, and dehydration. And if they were to get to that point, they could die from this.
So, while it is unfortunate that she brought home a kitten that has given a virulent strain of flu to her other cats, she needs to have them seen. Not doing so would constitute cruelty to these cats and she could be reported for this. At the very least, I doubt the rehoming facility that gave her the kitten would be happy to hear that this cat and the others are in this state. Therefore, she needs to be made aware that this is bordering on neglect and she needs to have them seen.
Please take care,