Situation: Player A has just started playing Pokemon X/Y and would like to have a battle with Player B. Player A has not gotten far enough in the game to get the Mega Ring, but has a Charizard with its Mega Stone. If they have a Wi-Fi battle together, will Player A be able to Mega Evolve his/her Charizard?
Logically, a Mega Ring is needed to mega evolve a Pokemon. Even if it has the stone, it won't Mega Evolve so Player A's Charizard would be unable to Mega evolve.
Hope I helped!