Can the game "asphalt8" be played on iOS and Android together?
According to "Diana FromGameloft" on the Gameloft forums:


  You and your friends have to play the same version of the game on the
  same platform/device in order to see each other.


Also "Gameloft_Ryan" posted on Windows Phone Central forums to say:


  You can only play cross-platform multiplayer with W8 and WP8 users.
  There isn't cross-platform for Android and iOS.


So unfortunately it looks like Gameloft haven't included cross-platform multi-player in Asphalt 8, so you and your friend won't be able to play against each other.