Should a patient of herpes zoster who successfully take medications for a month (pain and rash disappear) avoid contact with a diabetic patient and what are the precautions when dealing with each other ?
According to the CDC it depends if the virus is active or not, and they define active as:
A person with active shingles can spread the virus when the rash is in the blister-phase. A person is not infectious before the blisters appear. Once the rash has developed crusts, the person is no longer contagious.
The CDC also comments on transmission risk of active shingles:
Shingles is less contagious than chickenpox and the risk of a person with shingles spreading the virus is low if the rash is covered.
Here's the CDC info that also includes recommendations for preventing transmission of shingles.
They don't mention any specific precautions (other than the general ones on their site) for diabetics.