Theories about cheating are plausible and seem most likely to be true, but. At their first meeting, MC “shared his deepest secret.” Is his deepest secret that his girlfriend was unfaithful? A completely standard situation, nothing secretive about it. Or did MС, in the interval between the situation with Jacklyn, thoughts of suicide, and meeting Mei, pull out of his pocket information that was completely unrelated to these events? Mei also mentions the situation with Jacklyn, I think as an example to explain why she ran away from the camp. I mean how did she connect cheating ex and runaway from camp?
Something else happened. Something that we don't have enough information to explain yet. Right now, the only explanation I can think of for what happened is that Jacklyn was already in a relationship but was cheating on her boyfriend with MC. Nowhere does it say how long they had been dating, only that they had been friends for 14 years. Her parents also don't mention anything about the relationship. Perhaps MC and Jacklyn became close during tutoring sessions. This could also explain Jay's words on the pier about the two stolen girls.
Sorry for the long text, sometimes my mind does this kind of shit(