Libby, I have been where you are now, and I am so sorry for what you are going through. But I hope you will act quickly to minimize the damage. I am very concerned both about your husband saying that he is "choosing the assistant" over you and the fact that she is manipulative and taking money. My advice is to get to a lawyer, fast, and without this con-artist or your husband knowing that you are doing so. A divorce lawyer or eldercare lawyer will have experience with this scenario of a person with dementia being financially abused. Please do not underestimate how low these con-artists can go.
My husband was also involved with a financially manipulating person whom he insisted was just a "friend". I think if I had not found out, she would have talked him into divorcing me in addition to the money she "borrowed". My husband insists to this day that she was a "good person", and he could not admit or see how she was using him. We actually separated because of it, and thank goodness I insisted on a generous settlement, because after 18 months apart my husband was sick, broke, and his "friend" was nowhere to be found once the money was gone. My husband had to rely on what I had managed to preserve. This happened before I had any idea about FTD, so I had no other options than to legally separate.You have the advantage of knowing he has FTD, and I hope you realize that this person is totally taking advantage of your husband, and you have to protect both of you.
I would also recommend you ask the lawyer about writing a letter dismissing the assistant for financial irregularities, and get an order to prevent the "assistant" from entering your home, and freezing your husband's assets. You need to get her (and maybe all the other hangers-on) out of your husband's life, and assert your right to your home and your share of assets. With the FTD, he cannot see the ultimate consequences of his reckless spending and rejection of you.
If your husband has already been diagnosed with dementia you can invoke your power of attorney, but a lawyer will advise you on your options. I know how hard and horrible all of this is, but it will be easier once you have control of your finances and home. Good luck!