There are many types of dementia.
Drugs for one type(Alzheimers) can have an adverse effect on another type(Lewy Body).
The first is a 5-10 yr. forgetfulness slide and the second is 3-5 roller coaster mix with paranoia and shakes.
Diagnosis is important.
There are blood tests:
https://emedicine.medscape.com/article/292105-workup
There are also visual tests:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3493005/
Beware of the sundowners syndrome: Some people will decide to go walking one way or leave the kettle on.
Don't take things personally. It's often the disease talking, not them.
People will hold onto their dignity in many irrational stubborn ways (to us) because it's the last thing they have.
Keeping busy with things they can still do is most important.
Be there for them, and cognizant of the staff who deal with it full time but may be limited themselves.