It is safe to undergo dental treatment after three weeks since she had a uneventful recovery.
Since she is a diabetic she should maintain her blood
sugar within the permissible levels to prevent further complications in general.
Tooth ache needs to be investigated.
Please visit a dentist and make an x-ray to see whether it is tooth issue or a gum issue.
Dental visit does not cause any black fungus.
Having a lot of infection in the gums and tooth and low immunity and
diabetes can predispose her, so getting her treatment is a priority.
Next Steps
Warm salt water rinse and hygiene practices to be followed
Can take pain killer as needed, not for more than 2 days.
Health Tips
Do not delay as infection may increase