How could we fight a denied disability case?

Opinions of your mother's doctors are medical opinions, and disability is a legal concept. The doctors who treat your mother are trained to diagnose illnesses and treat them. The in-house doctor is trained to read the reports, check them against the agency guidelines, and decide whether the case fits into the legal parameters of disability. It does not mean that the in-house doctor is necessarily right, and you can fight against the determination. Before you can take the case to court, you have to do an appeal within the agency. Your mother was (or will be) given instructions for filing an appeal, but you really should think about retaining an attorney who specializes in disability law. Such specialist would evaluate the case and advise you whether your mother should do the appeal or a new case should be started instead. Whatever needs to be filed would be better prepared and have higher chances of success. Having an attorney at the hearing of your appeal before the administrative law judge can change the outcome of the case. And many of disability law attorneys work on contingency (which means that they get paid only if they win the case).

Are you speaking about on the job injury disability [Workers' Compensation], state disability, social security Disability, or what? ?The rules vary among the systems and vary from state to state except for SSI. Please re-post with the additional information, what your mother is trying to get [time off work, disability retirement, etc.], why specifically the employer's Dr. is refusing to agree [sounds like WC] and what right they have to be involved in the decision, etc.

I do not answer disability questions. This is not an employment law question.

Yes. She should consult a personal injury lawyer, or a workers compensation lawyer as soon as she can.

Get a competent Social Security lawyer. The test is: can she engage in any gainful employment?

I would have to see her disability policy and the reports of the doctors to answer your question. I recommend you consult a lawyer that handles disability insurance cases. Good luck.

It is possible that your mother may have a case, particularly if there are medical professionals that agree that she should not return to work. Your best course of action would be to call an attorney that handles accident cases to see what your options are.