I would contact an attorney even though you may be able to handle this on your own, doing it without an attorney might cause her to fight it and you already have an attorney for the other matters don't you?
In California, the grounds for annulment include but are not limited to:
Entering into the marriage as a result of fraud, force or duress,
which seems to be the grounds that apply to your father, however, what seems fraud to you may in fact not be fraud as unsavory as she may be or as her actions may seem. These marriages may come about as a result of a person exercising their right to make their own decisions about their care and or disagreements between the dying parent and their children. The wife may have entered into the marriage in good faith, with an agreement to care for your dying father and left to cut her losses rather than deal with the children, for that reason, divorce may be a simpler process.
Important things to take with you to the attorney are any records, report from the investigation, contracts, evidence of the proposed will etc. names of those attorneys, evidence of your father's intent to end the marriage and documentation of the date of separation (the date she moved out).
I answered this question in good faith, based on your post, then went back and looked at you previous posts and it seems there is some conflict in your story.
Please clarify your post here in light of your previous posts and advise already given.
https://forum.freeadvice.com/showthread.php?t=198498
Grandpa needs help
What is the name of your state? California. My 73 year old grandpa has been diagnosed with a terminal illness and been given only 1-2 months to live. He is currently on medications for his illness, plus he has been on anti-depressants for years and he drinks too much. He has begun making irrational statements towards family members he previously had a good relationship with. He was married for 48 years to my Grandma who died 3 years ago.
He claims he is going to give his entire estate to woman he just met 8 months ago. She is 51 and worked at a store he frequented. He says he wants to marry her and leave her everything. Of course she is all for it, because she has no home or even a car of her own. Grandpa has been taking her shopping, spending quite a bit of money on her and he just bought her a ruby and diamond "engagement" ring with matching earrings. He says they will marry at the end of this month. We think that because of his medical problems he is not able to make the best desicions for himself, and he has been unduly influenced by this woman. His existing will says that his 3 children will split all assets equally. Should we try to protect grandpa from himself and this woman by filing for power of attorney, or conservatorship? Would this protect his assets? Could it stop the marriage? What does the process involve?
How can your father be 70 and your grandfather 73 and in the same situation? While your father may have a problem, you don't tell the truth, even so, go to an attorney, the advice is the same, and tell them the truth from the begining!