Someone telling you an event happened is not evidence that the event happened.
If there is evidence that shows the event happened, then it is reasonable to accept that it happened.
"Evidence" is something that can be independently corroborated, tested and verified. It is not simply a claim.
"Evidence for a proposition is what supports this proposition. It is usually understood as an indication that the supported proposition is true...."
Evidence - Wikipedia
If person tells a story, it indicates that the proposition is true, which is why it can be called evidence. We wouldn't have the story, if it would not have happened. However, I understand it is not very good evidence, because person could just lie, or may have misunderstood.