Urgent- issue with forgetting in salah

Assalamu alaikum, I have a question about salah, I would appreciate it if you could please answer this question as soon as possible since it's been causing one of my close relatives a lot of frustration. I have a family member who has an issue with forgetting what he has done while he is in salah. The situation goes something as follows: after finishing a rak'ah, and after finishing Surah Fatiha of the next one, he finds that he can't remember if he did one or two sajdahs from the previous rak'ah. Or, upon reaching sajdah, he realizes he can't remember if he said Surah Fatiha in the beginning of that rak'ah. So my question is: what should be done in this case? Could you please answer what he should do if, while praying, 1) he forgets if he completed a fard action or 2) he forgets if he completed a wajib action? Also, what should happen if he experiences such doubts about what happened after the salah is over? In almost all such cases, he cannot reach a conclusion on what was likelier to have happened. A very important side note: in the side of my family which this relative is part of, there is a history of dementia/Alzheimer's (although he has not been diagnosed as such alhamdulillah), and this family member tends to forget other things beside matters related to salah throughout the day. Jazakumullahu khairan.