It would be best if she visits the dentist at the earliest for fillings or root canals, if need be. That would restrict the spread of decay. Diet modification ( which you have already initiated) and improvement in oral hygiene habits would also be necessary.
Next Steps
Pit and fissure sealants on vulnerable teeth and topical fluoride application as a preventive measure
Health Tips
Xrays would be necessary to gauge the depth and extent of caries( decay) in the affected teeth