SuttaCentral
Let’s say person A read all the books etc and knows theoretically about the 3 Noble Truths.
Person B, listened to a lot of true Dhamma, got into a 2nd jhana or so, and saw the reality of the 3 characteristics.
Person B gets alzheimers. How can we be confident that the experience and reality seen indeed irreversibly starts the end of the cycle of rebirth?
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To me, asking this question is the same as asking, “How can we have faith in the Dhamma?”
I think this is where understanding the principle of specific conditionality might be helpful. This principle says that one exists only to the extent that the other exist. In short, it is the cause and effect relationship just like there is rain only if there is formation of rain drops in the atmosphere. Similarly the opposite.
If B has experiential knowledge of the three characteristics it must co exist with wisdom which is like seeing the light at the end of tunnel. This is because B also looks at the opposite which is not knowing the three characteristics and his current state. So, B only needs to understand his current suffering in relation to his ignorance of the three characteristics to be certain that end of the cycle has begun.
With Metta
The kamma produced before they lost their memory will continue to come to fruition when conditions are suitable, that is in the next life when their faculties are normal.