What should we do if we suspect my father is the potential victim of elder financial abuse?

You see an attorney immediately. NOW. Protect your father. You file for conservatorship before she does.

Definitely see a lawyer NOW. There are things that can be done, even if you don't have obvious proof. A few years back Dad developed a relationship with a woman who was much closer to his own age, and that seemed like a really nice situation for him, until little things started to make us suspicious. Things like inconsistent answers about where she lived and if she had family. Nothing in itself a red flag, but odd none the less. I spoke to the lawyer, and since I was already helping handle Dad's bills and medical issues, it wasn't difficult to set up his accounts (with laywer and bank help) so that Dad couldn't take money out alone. Moved the bulk of the money to a separate account, supposedly to earn more interest. He needed me or one of my brothers to sign with him for it. The rest was in a checking account with very limited amount, that he could access easily. Low and behold, the instant the lady friend learned that the money was no longer in only Dad's name, (and would be left to account survivors should he pass away), she dissappeared from his life. ...It was sad to see him miss her, but it was better than seeing him get taken advantage of.,,,,,,See that lawyer.

You can talk to an attorney all you want, but you will need evidence of abuse before you can force her out of the situation. Talk lovingly to your dad about your concerns. If he is not incompetent, you have no legal right to mess with his financial affairs--nor does the new woman. If he wants a new companion, maybe he feels like he is taking advantage of this young woman, not the other way round.
If you believe he is being duped or scammed, you can speak up to the both of them with loving concern for his well-being. That warns her that you're watching and reassures him that you care--hopefully about him and not just your potential inheritance.

1. Follow Pamstema's advice immediately. 2. Start keeping a private log of everything you observe this woman doing, positive and negative. Where does he live, alone? With her? With you? The attorney you hire will tell you what documents and information you need to file for conservatorship and guide you through the process. I did that for my mother after my father died suddenly,

Talk to your father and voice your concerns. But, if he is competent, he has a right to be with whomever he chooses. My best friend's (from grade school) father met a woman at their church who knew he had terminal cancer, but asked this younger woman to help him. They eventually married, and she got the house and my girlfriend (who is a twin) tried to fight it in court and they did not fair so well. So before you entangle yourself in your father's affairs, beware.

How is Dad's health? This may be crucial in gaining any support for your concerns and trust from Dad. Who has the ability to see any of Dad's financials? How did this woman get into the picture? How has your relationship been with Dad in the past? You will have to be sensitive with Dad on this issue as he might not want to be shown he could be making a mistake.
I could go on and on with questions but the reality is you need to see YOUR attorney as mentioned by Pam.....or a different attorney one from Dad's.Look for an attorney who is Elder law etc. And when you do take a list of your questions...be brief on the trivial details and get right to the point.

Jinx, no need to yell at me! The OP ask a question and I was giving my opinion as well as some facts. I live by certain values and I have every right to express them according to my first amendment right and I'm sticking to that because families are divine design and God made the rules about how things are supposed to be and I live by them!

Call Adult Protective Services. Share your concerns. There may be other reports regarding this person. Also contact an attorney. As much as we hate it, our parents are allowed to make crappy decisions. Hang in there, and stay close with your dad.

APS was useless in my situation. They told us that as long as he is "a willing victim", there is nothing they can do. It might be different in your state, though.

As you learned, without evidence of abuse or incompetence, there's not much you can do without your dad's consent. Please talk to him about your concern for his and your brother's financial future. Offer to take him to a lawyer to review his estate plan. Maybe he would want to set up a trust for your brother.
Be open with him about your suspicions. Maybe he has seen it in her, too, and is already prepared for her to try something.
If he is not willing to discuss your concern at all, then bring it up to her in his presence. Let her know that you're concerned and that you're watching. If she is just a gold-digger, she may move on to easier pickings.