It is absolutely NOT too late for your mother to consult with a geriatric psychologist and/or psychiatrist. You may have trouble finding one, however, as, in many states, there is a shortage of mental health professionals who specialize in treating the elderly. One way to go about finding someone would be to ask the agency that provides her current care if they can recommend someone. Another way would be to look on the American Psychological Association's website- www.apa.org and see if they can locate someone near you.
You are probably right that your suggestion that she get some professional help might be met with reluctance. Usually however, a good geriatric psychologist can form a good connection and break down this resistance very quickly once they meet. What you might try is framing the meeting as a way to help her get the care she needs as you know she is unhappy with the current care provided. This way you are acknowledging her current dissatisfaction which, hopefully, will make her less defensive and upset about finding someone new.