On Memoirs and Lying
The American reading public loves a good memoir, and does seem to forgive authors whose memoirs are revealed to be a pack of lies or based thereon (e.g., James Frey, although Oprah might not be included in the population of forgivers; Laura Albert/JT LeRoy; and most recently, Margaret Seltzer/Margaret B. Jones).
Be that as it may, there is supposed to be a rule at work with memoir: don't lie. The interpretation of that rule can be a bit flexible when it comes to dialogue and other details that add richness to a moment, but the fact that the moment did actually happen (particularly if it's a major moment in the story), should be indisputable.
Some authors do not include anything they do not recall, meaning they don't concoct dialogue they don't remember. Others believe a memoir is flat without dialogue and manufacture it accordingly. But they don't change the net result of the exchanges; they're just trying to bring the story to life and they believe dialogue injects that life.
Finally, our memories are shifty creatures at the mercy of the baggage they're embedded in, and all we can really attest to as authors is: this is my recollection of the events, people and conversations. We do our best to verify what is practical and possible to verify, but some of what is included in memoir gets a free pass from authentication.
How much gets a free pass is the question.