Yeah, bit of a rookie mistake on my part. (To be fair to DPC, the MC does specifically say he "saw" Zoey got a new boyfriend in his narration.)
I'll never doubt DPC's commitment to avoiding NTR again! Though I am now fascinated to see how he handles the vixens.
We know he must have seen Emma's cluck since there's no other way he could have thought Zoey had a boyfriend. It's possible Zoey had a copy of the pictures herself (they were supposedly something for Zoey to reminisce on when she got old), in which case she might have sent the first one (with her top on) to the MC herself. That would explain why the MC 'saw' it in his narration to Bella even though Emma only posted the topless pic.
As for the timing, I suspect that's being kept intentionally vague to avoid the age issue. She likely left during the school year, so somewhere in the October to May range one year before the present. The MC said he hadn't got a message from her in six months, and that was during the second week of classes. So his last contact with her was in ~February. But in the Interlude Zoey explicitly says she stopped writing Evan daily after the first few months, and then at least a couple more weeks passed before their final message. That moves us back to the November range at the latest.
So I'd say she left somewhere between October and November of the previous year. Of course, this makes her flashback sex scenes with the MC highly suspect, but we'll just shove that quietly under the rug. (That, or her total relationship with the MC was even shorter than his whirlwind romances with the main girls!)
More likely Zoey got help back, I'd say. I suppose it's also possible she was no longer 'in' school and would have completed her trig class at home in much the same way she did in San Diego.
We don't *know* that the cluck was 6 months ago; it could have been even longer if it came after the MC and Zoey stopped messaging. I think it's likely that was what finally prevented the MC from reaching out further, but we can't be absolutely certain of that.
Either way, however, it really makes me wonder why Zoey didn't contact the MC in the next six months once she realized how much she missed him. She conveniently waited until he had a new phone with its retconned new phone number.