I'm cross posting this to sci.med in hopes that Steve Harris, a geriatrician,
or some of the other physicians in that group will read it and reply.

Elderly people can become demented from the drugs they take -- they are
far more likely to have this side effect than younger people, just as
they can become very confused from a relatively low fever, unlike
younger people.  They also often take many different drugs, with
unpredictable interactions.

Try to get a referral to a geriatrician or an internist specializing in
the elderly.  Sometimes very carefully taking the patient off the drugs
in a hospital setting can sort out whether this is the cause of the
dementia, and a better combination of drugs can be worked out.  Or the
specialist may have other ideas about ameliorating your partner's
problems.

I have no medical qualifications.  Best wishes to you and your partner.
Living with a demented person is extremely stressful, and seeing
someone you care about deteriorate like this is harrowing.