The condition your mother is suffering from is called as Post-herpetic neuralgia
This pain may stay for many months even after the lesions have subsided.
Pain killer tablets are usually of not much help.
She needs to take a nerve stabilizing agent like Gabapentin
for few months.
Although this will help, but there is no permanent cure to this condition.
Don't worry about the nerve damage
...there will be only pain & nothing more.
Wishing your mother a good health.
Thanks & take care.