Treatment of fungal infection of toe nails requires the longest course of weekly anti fungal tablet and daily antifungal ointment - almost a year
Without seeing it, we can’t prescribe medicines, coz it could be onycholysis/Hansen’s/ anything else other than onycomycosis(fungal)
Next Steps
Repost the question with pictures of the mail or consult online
Health Tips
Your fathers diabetic medication detail is also important and must be shown. to avoid interactions correct anti fungal to be given