I'm so sorry you are having to go through this. There can be physical causes of dementia - a urinary tract infection is a comon one. Plus, there are common aging problems (inability to retrieve words) that are similar to the early signs of dementia. So, one step I would recommend is for you and your dad to talk to her doctor (preferably a geriatric specialist) about your concerns. He may not be able to disclose things about her state, but you can give him information that will help him evaluate her condition.
Both my mother and my late father had various forms of dementia. My dad started with forgetfulness, missing appointments, and not being able to problem solve. He remained very sweet and could pay bills but was easily taken advantage of. My mom's started after my dad's death when she had a psychotic break. After treatment, she was better but still had short term memory loss, repetitive stories, intense negativity. She sees a geriatric psychiatrist and is on some psychiatric medication which controls her extreme anxiety. It's been incredibly difficult for me.
BUT, everyone is different and there is help available. Let us know how things work out.