>Once the conversation wraps up, Lysandre will leave. You should do the same, too. As you leave the café, you'll receive yet another Holo Clip call from your friend, Trevor, who tells you that everyone is going to meet on Route 14.
Head back to North Boulevard (using Fly to get there works well) and then head into the gate to Route 14!
According to Azurilland's walkthrough, the holo clip should trigger once you leave the café. Perhaps you forgot? Have you tried going to Route 14 yet? Once you get there Trevor will compare PokéDexes with you and Serena will want to fight.