How to clean a nail cutter that was used by someone that has sick nails?

Just buy a new one and hide it from him.

Ahh. The Funges Thingy. Douse the clippers with isopropyl alcohol, hold it under a torch for 15 seconds. When it cools wrap it in heavy duty foil, wrap in duct tape and bury it at least 1 foot deep. Can't be messin with no fungs. Sorry, I mean Funges

Twice in bleach. Once in water.

Give the nail clipper to your dad and buy a new one already!

I would wash with soap and water and then pour rubbing  alcohol /isopropyl alcohol over them and let dry.

To sanitize you would need to leave it in boiling water for about five minutes 

Any disinfectant should do the trick. Just clean it with rubbing alcohol. You could also use boiling water. In the future, I suggest you don’t let other people use them. I wouldn’t want someone with toe fungus (or anyone really) to use my nail clippers. It’s perfectly safe if you disinfect it before using it, but it’s just kind of a gross feeling to know that someone’s been using your nail clippers on their toenail fungus. Let your dad use his own clippers. Certain personal hygiene items are best not to share, such as nail clippers, nail files, razors, toothbrushes, hairbrushes, pumice stones, hair trimmers, tweezers, deodorant, makeup, bar soap, etc. 

Use the hand saunter you have been using on your hands regularly because of covid.  

Placing them in boiling water is good. Wiping them with a solution of bleach and water after should get rid of whatever is left. Do you have hand sanitizer? Give them a squirt of that. Your nail clippers are now clean and sterile.