I think between Meg ignoring him for a while, and his girlfriend telling him lies about how Meg couldn't stand to be around him anymore that it could work that way perfectly. By the time Meg would want to tell him because she misses her best friend, it would be to late and there would be too big of a rift.

I think it would be more like a childhood thing, they talked about it a lot until they were probably in second or third grade then it just became the thing of the past. So really, them forgetting would be really easy by that point. Not to mention their memories of their "dreams" would be so sketchy when they were young because the worlds would not have been as close as they are now.