A dementia patient can live in assisted living as long as they are just in the early stages of dementia.
If the patients dementia is more severe then they will need to live in a regular nursing home facility where they are more taken care of and have better staff available for the dementia patients needs.
Dementia patients can sometimes get dangerous and cannot really be left alone for too long of a period so in assisted living they would sometimes be alone in like an apartment which is not ideal for a dementia patient who has more severe dementia.
My Grandma lived in assisted living in the early stages of dementia but after 5 years of being in the assisted living her dementia got worse and she had to be moved to a regular nursing home for her remainder years.
She was getting violent and leaving the stove on etc so it was getting dangerous for her and the other assisted living patients to have her living there.