Best remedy for toenail fungus for my 92 year old mom?

Podiatry. Complete eradication of a fungal infection of the toenail generally involves removal of the nail and a prolonged course of oral antifungal meds that are very toxic to the liver. Seeing a podiatrist to keep the nial short and clean is often enough to prevent any comnplications related to onychomycosis(toe nail fungus).
No real cure. At 92 i would not recommend oral meds. I would look at topical or laser treatment.