C diff cleaning. What is absolutely critical to clean?

You're off to a good start! I'd add kitchen, including appliances. Re: leather chair - there are products specifically for leather which are most safe

I guess the thing is...the second person in the house could be the person without the best hygiene. We're assuming that the person with the c-diff was the one that wasn't washing their hands well--but it could be the other person, communicated to her via bathroom taps or something. Or picked up in public.

the CDC.gov web site has some great information, for lay people and professionals.
The cleaning advice from Katefalc is excellent....a couple of additional suggestions.
Alcohol gel does NOT remove C. diff organism from your hands, or alcohol wipes on surfaces. A bleach based cleaner will.

Do wash your hands with soap and water and use paper towels to turn off faucets and dry your hands. Put on gloves. Try not to touch anything until you are in her bedroom, and then do what she needs.

All of the clothing that she wears should be placed into a trash bag and loosely tied up. Put bag in garage or out on the steps until you can wash and dry the items.
If she is incontinent (very common) bag up her trash the same way, and tie tightly. Can go out with regular municipal pickup. Same for gloves, used bath wipes, TP, etc.
A waterproof mattress cover will help protect the mattress if that is important.

Think about wearing an extra shirt or sweater when you are in close contact with her, to prevent transfer to spores to your clothing. Then remove the extra garment, turn it inside out (dirty sides folded inward, the clean sides on the outside), and leave it outside of her bedroom or bathroom.

Do wash your hands with soap and water and use paper towels to turn off faucets and dry your hands. Put on gloves. Try not to touch anything until you are in her bedroom, and then do what she needs.

Good luck!

Use bleach wipes on door knobs, fridge and appliance handles, cabinet handles , counter tops and faucet handles. I would wipe down the seat and arms of the leather chair with antibacterial soap in hot water then wipe dry. It is contagious in the stool so don’t freak out, just good handwashing, hand sanitizer and common sense .

Sad people with C diff are treated like they have leprosy

Please while you are taking care to clean and sanitize make sure you are wearing gloves.
While you are cleaning Gloves. If you leave the room you are cleaning remove the gloves and toss them out and wash your hands. When you return to cleaning glove up again.
Try not to touch your face while you are cleaning
And as long as you have them handy wear a mask. Change masks like you change gloves.

The kitchen would be a good spot to disinfect, too. Is your LO able to wash and sanitize after using things in their house? Strangely enough, I remember plants as being a spreader of c-diff from back when my grandmother had it. I think that's because it can live in soil for a long time.

Replace things that are in the bathroom that you can, like her toothbrush. Wipe down everything she might have touched (eyeglasses, brushes, pill bottles, etc). Throw out all 'old' mops, cleaning rags, etc., that she might have used in the bathroom & house. Clean all the handles, switches, knobs etc throughout the house. If the bed is ok it might not hurt to get a mattress cover anyway. Does she drive? Wipe down the car handle & steering wheel. Kitchen: Run her silverware, glasses, utensils, and dishes through the washer along with anything else you think she was handling a lot. Wash the dishtowels. Consider putting paper towels in the bathroom and kitchen if hygiene is an issue, rather than having her use cloth towels. For electronics, like phones, mouses, keyboards, etc., dampen a cloth with your 9:1 water/bleach solution and wipe them down. They should be slightly damp but not wet. Let them air dry and then you can buff any haze off of them with a 50:50 water/alcohol dampened cloth. If she has forced air heat/AC then replace the filters (Amazon has good prices). Consider if there's a need to clean the carpet. The leather--that's a tough one. Maybe try to do a couple of passes with the bleach solution where you'll just dampen a test spot and let it dry a few times to see how it does? Go get 'em!