Naomi,

The ratio of salt to water is minimal. Just a small packet (like you get with take-out food, in a big bowl. It'll help stop the water freezing.

One thing your mom might want to do is get an 18 or 20 gallon plastic bin (not transparent). Flip it over, lid side down and cut a hole in it large enough for a cat to get through. Line the bottom with newspaper, straw, an old blanket or towels and, if available top it with a plastic tarp. It will act as a shelter against wind and cold. You can use a smaller tub to set up a feeding station too.

Good luck! And keep us posted.