As the wounds are not superficial they will require antibiotic treatment. Topical treatment will not have any effect on systemic infection and will likely result in septicemia. Can i suggest some pain relief , Metacam , a pain relief medication for cat and dogs has proven effective for turtles (prescription med from vet)
Necrotic and infected tissue will develop and should be removed , with topical treatments applied ,this is often to painful and needs to be done under anesthetic as is generally needed with serious shell rot infections.
If the owner is not willing to pay for treatment then they should as least pay to humanly euthanize the animal .