My mom filed for divorce before she was diagnosed with severe demetia and declined rapidly.

Your mother isn't expected to understand much as her brain has changed, sadly.

You should retain an elder law attorney.

If she has been declared incompetent, she could not sign legal papers, unless it is for guardianship/conservatorship. The judge will recognize her inabilities to answer simple questions. Get another attorney, preferably one who is competent in medical cases not just domestic relations.

I'm afraid the train has pulled away from the Station here and it's too late. I would agree with the Lawyer 100%. You know that Your por Mom is dying, so why would She even consider divorcing Your Dad.
Your Mom will need You by Her side all the time now Terrypa and She's so Blessed to have You. One Little Tip though, it's best not to mention the dementia to Your Mom, just pass it off by saying " ah sher Weer all forgetful.

I agree with Curchmouse. My husband wanted to divorce me for a number of reasons and for no reason at all. And then the next day, things where fine, that is when I realized something was wrong. Five years later, he adores me, but doesn't think we are married all the time. Sometimes he thinks we should get married, because he says I take such good care of him and other times he wants to see the marriage cert. to prove we are married. He was going thru a stage of Alzheimer's and we didn't realize what was happening.
You need to find out why your mother wants a divorce, does it make sense. Sounds like her husband is not your father? How long have they been married?

Now that your mother has been 'declared' incompetent she has, if you like, exchanged her right to autonomy for a right to protection. You, if you are awarded guardianship by the court, will become responsible for ensuring your mother's best interests. I suspect that the lawyer's phrasing was clumsy: not so much what your mother wants, as what is in her best interests. And, all the same, what is in her best interests may not be what you would like to see happen. That's the distinction.

When you say her husband is manipulating her because he "doesn't want to pay", he doesn't want to pay for what? For the costs of divorce? For supporting himself? For supporting her?

One final thing: you don't say how long it was before she was diagnosed with "severe" dementia that she filed for divorce. Well, severe dementia doesn't just happen overnight. She will have been ill for some time leading up to it. This matters, because it is possible that dementia was already at play when she began to consider divorce. Just something to bear in mind.

After becoming your mother's guardian and conservator, discuss this matter with the guardianship as well as the divorce lawyer. Laws vary from state to state. In some states, the guardian of a legally incapacitated person can sue for divorce on their behalf. The amount awarded a spouse in divorce also varies from state to state and situation to situation, perhaps the biggest difference being between "community property" and "[English] common law" states. You may also want to talk with an elder law attorney (see www.naela.org for those in your area) about how the divorce and the distribution of assets and allocation of income in the divorce decree would affect your mother's eligibility for Medicaid, Most people eventually need Medicaid for nursing home care.

Where will your mom be living? Is she in any danger if she lives with her husband? Is she ready for nursing home or memory care?

What kind of difference do you think it will make in your mother's quality of life if she stays married vs getting a divorce?

The lawyer is giving you good advice. The husband will end up paying anyway, because the courts will award the wife her fair share of everything.
Sometimes couples seek divorce when one of them needs a nursing home, thinking they can save assets from MERP. Well, they can't, pure and simple. Medicaid knows all the games people play. Sometimes old ladies give everything away and go into a nursing home and apply for Medicaid. Guess what. Medicaid won't pay either, so the NH has to sue the kids.