Causality & memories

I was reading "Friendship is Optimal" (a my little pony fic) and there was a discussion in chapter 8 that was really interesting.

The main character is in a simulated reality; his body no longer exists. He is talking to his simulated gf, who never existed outside the simulation. She tells him about her youth and he points out that the simulation didn't exist during the time she was talking about, and they discuss what it means for events to happen.

Gf tells MC that her memories are consistent with other simulated ppl and the environment they are in. MC asks if consistency of memory is sufficient to say that something occured and she says no. Gf then says that its also that any future memories she creates also are consistent; she knows the events she remembers had observable effects. In other words, if she her observations are consistent with an event, that is sufficient for her to conclude that it happened. MC thinks that this is not enough, since the events were never simulated in real time.

I would agree with the GF here. The reason the MC doesn't think the events happened is because he remembers the simulation not existing for very long before he entered it; in other words, he objects because he thinks his memory is inconsistent the events happening. Since his only reason to doubt that the events happened is because his memory is inconsistent with them and if those memories were changed, he would not have cause to doubt, I would say its meaningless to define some objective chronology of events independant of your memory. Regardless of if it exists, you wouldn't be able to observe it and it wouldn't affect your existence.

What do you think? Does the idea of an objective chronology of events make sense? Related idea: does it make sense to prefer reality over simulation when there is no observable difference?

I know there are ppl who would say yes to both those questions, but I think answering no makes much more sense. How could there be a difference between reality and a simulation indestinguishable from reality?