Can a man who is 91 and has dementia now be under the care of his daughter who was declared mentally ill & is under his guardianship?

If he fell while in "a home" I'm not sure why she had to take him to the hospital and not the facility. Or if one can call APS for someone already in a NH? Maybe talk to an ombudsman? Falls are very common in the very elderly and she might not have told the "home" that he had balance issues or wandered at night. If he is indeed in a NH then maybe the admin there would need to know but unless you have actual proof about him being her guardian, not sure they will or can act on your information. This goes for APS as well -- you need some sort of proof of something going on. I hope your concern for him is acted upon by the appropriate authorities.

In my opinion, no she cannot care for him. I agree, get APS involved. She is spending money that is needed for his care.

I don't know the OP's connection to the elder here. But an elder with dementia is a vulnerable person. As such, needs a Guardian looking after his interests. The elder could be at serious risk of financial abuse & neglect.

Without further info to go on I think I'd report to APS for investigation.