Should my mother tell the doctor my father doesn't always recognize her?
My father had a stroke a little over a year ago and was more recently diagnosed with dementia. My mother is his primary caregiver. My father often believes my mother is his mother and even calls her 'momma'. During times when he sees my mother as his wife, he asks her where she goes - referring to when he thinks she is his mother. My mother hasn't spoken to his doctor about this and because she doesn't want to upset him she plays whatever "role" is called for in any given circumstance. Should my mother continue with this role playing or do something else?
Yes, your mother is responding to your father in a way that's appropriate. People with dementia often confuse their loved ones for others. Correcting him every time he makes this mistake could be confusing and upsetting for them both; going along with where he is in the present may make their interactions smoother. If she's comfortable with doing this, there's no harm in it.
She shouldn't be shy about mentioning this development to his doctor, however. Because your father has already been diagnosed, this symptom may come as no surprise. But it's also useful information to help provide the doctor with a complete picture of your father and especially if it's a sudden or recent change, the doctor may want to rule out a medication interaction or other cause separate from the dementia.