Pokémon Amie does affect friendship, but it's much faster to just to get a massage, level it up (since he's got the EVs he wants it should be fine), and juice from the Juice Shoppe o.O
The only thing that Friendship affects in any battle is the base power of Return and Frustration, which will have the same power in Battle Maison battles as they do anywhere else.
In regards to the hearts you mentioned, you may be confusing Friendship and Affection. Friendship is increased through various activities including massages, leveling up, Vitamins, etc.
While Affection is raised solely through Pokémon-Amie. Additionally, the only Pokémon that requires any hearts of Affection to evolve is Eevee, although you only need two hearts minimum.
Carnivore said he (or Alfredo...?) were going to evolve their Pokémon o.o and there's easy ways of doing it. Games on Pokémon Amie can get boring after a while
Again, Friendship and Affection are two entirely different things. The only Pokémon that requires Affection to evolve is Eevee. All the rest need Friendship.
Chansey needs max friendship, which can be easily done through leveling it up then checking it's friendship with the lady in Laverre City. :) I can't remember what she says for max Friendship
