When you do, that player loses the game.
As a partner, Mai was only obtainable through 2 special events, lasting from: July 16, 2014, 10:35 (UTC) through August 26, 2014, 23:59 (UTC) (her normal version) October 8, 2014, 10:00 (UTC) through October 29, 2014, 23:59 (UTC) (her Orichalcos version) Players who spent Miniclip Credits in-game, before the maintenance period of a Wednesday during the first time period, were qualified to receive her in their Gift Boxes (which they did after the maintenance period of that applicable Wednesday)
Or just play them as a land if needed
their high playability makes them a favorite among players as well
Here's a breakdown of the main Pokmon TCG product types available in Europe: Booster Box (Display) 36 packs
POKMON CARD GAME sv8 027/106Common card