Some scholars have made sanad Hasan, and in the sanad there is a Man who narrates, for example so and so the son of so and so said, but this man is weak in memory, but He also saw what was being narrated with His own eyes, so there is no doubt that He remembers what happened quite well. So my question is, would the narration of such a man be accepted? and why would it be considered Hasan, and not saheeh?
If a chain of narration is downgraded from sahīh (authentic) to hassan (good) due to the weak memory of one of the narrators, then the condition you posed cannot exist. That is, it could not be possible that it is a situation observed with one of the narrators own eyes.
The situation you are describing can only come based on a hadith that describes an action, of which one or more companions would have observed.
The first layer of a hadith narration is that of the companions. All companions are considered both 'adl (trustworthy) and tām al-dabt (able to memorize). The reason all companions are considered trustworthy and able to memorize is that the Prophet ﷺ would accept the words of a single companion without questioning or seeking corroboration. Likewise, the Prophet ﷺ would send a single companion out with a message of his without the need to have another companion to remind the first one in case he forgot.
The second and subsequent layers, where the issue of poor memory comes into play, would not have seen the Prophet ﷺ do an action with their own eyes, as they simply were not alive at the time of the Prophet ﷺ. Furthermore, the quality of tamām al-dabt is that of the memory, and it is not a function of the sensory organ that experienced the incident (hearing, seeing, tasting, smelling, or touching).