I could see Dawn get upset when bringing up Paul because he forgot about her again lol. I mean if he does show up i think their will very minimal hints but it would be nice for them to stick with each other and argue. I'm hoping both of them go back together when they have to leave.
I can literally see it perfectly playing out in my head: Dawn, before going to Unova, runs into Paul while she's traveling through Hoenn and is like, "Oh, hey! How are you, did you beat Brandon?" and being all polite, but he's just like, "... Who are you again?"
As for the question, the most utterly plausible thing I could see happening is Cynthia mentioning Paul won the Battle Frontier to Ash and the others and Dawn commenting, "No surprise there; he's really strong!" and then proceeding to explain who Paul is to Iris and Cilan.