MIL who has lived with us for a year has started doing her laundry in the bathroom. How do I approach this delicately?

Maybe an accident and because it's a new thing, embarrassing. It can also be a step on the dementia ladder and she has forgotten how to use it.

It is awful watching our lived ones decline into dementia and the amount of things that they go through is gut wrenching. I still can't figure out if my dad knows or if he is clueless about his lapses.

I hope it was something simple like boredom and all is well.

I think that you have been given some good ideas about what might be the cause. Incontinence or forgetting how to use the machine. I hope you caught the down comforter in time to dry it properly before it became odorous. Whoo nothing smells quite like it.

I think a friendly approach to laundry day, maybe do it together, then you can chat and sort or fold.

I would also get her checked for a UTI, changes from one can be any kind of behavior changes, not everyone gets angry, some get more forgetful, loose balance, etc. Once you rule out any medical cause, then implement a laundry routine. I would check her mattress on the sly, this could be very enlightening.

duped post

She just may need reminding...
Mom today is laundry day let's get those clothes in the machine...
Mom this is a special comforter let's get in the car and take it to the cleaners....
Mom I have room in the machine for some of your towels let's save some water and wash them together...

You can also place a note on the bathroom on the mirror. Clothes go in the machine.
I have had to do this with my father.
Blessings
hgnhgn
You may want to check hygiene practices too.

My mother has been weird about laundry (and other stuff) for a couple of years now. She doesn't have Alzheimer's but she definitely has cognitive deficits. She is afraid of the machines themselves, including washer, dryer, dishwasher, thermostat, microwave oven, elevator, you name it.
We (sisters and I) believe that when she still in independent living she was drying herself off after a shower by using paper towels. That way, the bath towels wouldn't need washing. My sister the detective thought, hmm, she's going through a lot of paper towels, and, hmm, the bath towel never seems even wrinkled.
Mom complained one time about the bathmat being heavy. We think to ourselves, Why is she lifting the bathmat? Well, she had soiled it and tried to wash it in the shower. A wet bathmat is really heavy.
Mom gets upset that my sister, the close-by daughter and detective, nabs her dirty clothing items and puts them in the hamper for washing by the AL facility.
In your case, I'd bet incontinence and embarrassment are elements, plus perhaps forgetting how to do the wash.
My mother asked me quite a few times whether the pantiliners could be reused. No, Mom, no.
Even now, she wants to remove dirty clothes from the hamper and put them back in drawers.
UTI's come and go because of unsanitary situations that all of us have worked on to address. But she's 92, cognitively deficient but defiant. Getting her to change is not likely to happen. (But she always used to have excellent hygiene and kept her house sparkling; what happened to That habit?)
This is a brain problem.
Keep your ears and eyes open and think like a detective.

MIL needs more supervision. It must take her a good while to wash those things and then drape them all over her room. When she “ disappears”, keep a surreptitious eye on her. If you see her taking her clothes and bedding into the bathroom, follow her and in a cheery voice, say “Oh, Mom, you don’t need to do that! I was just about to put a load in the washing machine. I’ll do your clothes for you!” If she protests, calmly take the clothes from her and reassure her you’ll bring them back, all clean and dry. She should not be carrying a heavy, wet comforter around. That a definite trip hazard.

What you think is hurt feeling may be confusion on her part. If you keep the tone light but firm, I don’t think she will protest.

Could she have soiled herself and was trying to hide the evidence?

For the duvet, perhaps tell her you are taking all the bedding in the house for Spring Cleaning, and make sure it is thoroughly clean.

i have in the past washed a down duvet in the bathtub. It is possible, but is heavy work and needs to be thoroughly dried. I like to line dry them on a sunny windy day.

For her regular laundry, could you tell her you do not have enough for a full load and gather up some of hers to top up the load? She maybe losing the ability to manage the task, but trying to hide it.