Dog Has Been Being Sick, Drinking Lots Of Water. Went To The Vets This Afternoon, He Gave Her A Shot For The Sickness And She's Booked In Again For Tomorrow. Now Her Back Legs Seem To Have Collapsed. Is There Anything To Reassure Her Owner With?

Your dog's condition is progressing and I would not recommend waiting until tomorrow to seek further veterinary medical attention.  I assume your veterinarian ran diagnostic tests to determine the cause of your dog's illness.  I also hope your veterinarian explained the diagnosis and what the shot was treating.  The diagnosis is important for correct treatment and prognosis.  
If you cannot have her examined before tomorrow make her as comfortable as possible with blankets, accessible food and water.  Help her move around, urinate, and defecate with the help of a towel sling.