Hello @MonP
Dementia can complicate the treatment of any medical condition. Even in those without a mental health diagnosis, behaviours and personalities change. I do not know if a PD contributes to the risk of developing what are seen as the more challenging aspects of dementia.
Clear communication in relation to the treatment of any medical condition is necessary - not just psych, do you have medical POA? Medication reviews are often helpful to ensure no untoward interactions between prescribed meds. There is a lot of controversy about the use of antipsychotics in patients with dementia (most carry a black box warning for use in the elderly) and these are the RANZCP guidelines.
https://www.google.com.au/url?sa=t&source=web&rct=j&url=https://www.ranzcp.org/Files/Resources/College_Statements/Practice_Guidelines/pg10-pdf.aspx&ved=2ahUKEwimm7CE_PzdAhVHE4gKHaUqAY0QFjABegQICBAB&usg=AOvVaw15yUY_ErB7zNnyLehpyexe
Help and support is available through Dementia Australia, their website has some excellent information as well as carer support options.
https://www.dementia.org.au
Is it BPD or another PD your father has? The following is a link to nhmrc guidelines regarding the treatment of BPD.
https://www.google.com.au/url?sa=t&source=web&rct=j&url=http://bpdfoundation.org.au/images/mh25_borderline_personality_guideline.pdf&ved=2ahUKEwjkrpSS8_zdAhULUd4KHWVXChYQFjACegQIBxAB&usg=AOvVaw1t33Ak6kPpAXueVB3YHKwJ