My late husband's daughter was active teacher and businesswoman. now 68 and signs of cognitive impairment who should talk with her?

Anyone can: Dementia is a difficult thing to experience and address. It can be very subtle, but if it is now noticeable, the chances for her to be a danger to herself are greater (driving, leaving something unattended on the stove). Some reversible things can cause dementia, so best to be seen sooner to check into them. A physician can help discuss the problem and do a detailed exam to see how bad it is.

Neurologist: She should be gently guided to see a neurologist for a "check-up" and possible strategies on improving cognition (including but not limited to several medications). Someone from the family should certainly accompany her to the appointment.