Alzheimer's/ Dementia mother-in-law is not eating solid food. Recomendation? Why is she not eating?

She may have forgotten how to chew, or as some have suggested she may have a sore in her mouth or a bad tooth bothering her. She also may have trouble swallowing, which she can be tested for, and in which case she would have to have all her foods pureed and her drinks thickened with Thick It, as otherwise she could aspirate, which is very dangerous, and could lead to death. Either way she needs to be checked out thoroughly by her Dr to see what the issue is. Best wishes.

A speech therapist can evaluate her swallowing capabilities. Ask her medical practitioner to order this. They can come to her in many circumstances. I agree with not forcing her.

There is a difference between not eating and not feeding herself, which is it? My mom gave up attempting to feed herself almost everything years before she died but always opened her mouth like a little bird when I fed her (I vowed that I would never try to force her to eat, but she almost never refused). Along the way she also began having difficulty with anything that wasn't moist and smooth as well as thin liquids like tea and water, she lived on a completely pureed diet with thickened fluids for at least two years, probably three.

Depending where she is in her diagnosis, there are a number of reasons why this may have happened to her.

Was she eating better and then stopped, or did she gradually slowdown in her eating behavior?

Have you looked into her mouth to see if she has a large cavity or broken a tooth or is part of a dental appliance bent or broken? Any visible sores in her mouth?

Has she had issues with choking or coughing while eating? Does she seem to be uncomfortable when swallowing? Will she drink shakes or puréed soup? Are there any particular textures that she is avoiding?

My mother stopped eating about 6 weeks before she died, and would not allow me to feed her. The residence staff told me to leave her alone and not try to force her.
It sounds like your next step should be her doctor’s opinion.