Is pocketing an opponent's coin considered a foul?
According to the Laws of Carrom 125 


  When a player pockets the C/M of his opponent knowingly or unknowingly, the Umpire shall stop the player from playing further. If unnoticed by the Umpire or by the opponent, the C/M pocketed shall be considered to have been properly pocketed. The turn of the player shall be lost as and when noticed.


So, while the action is not itself a foul, you do lose the turn, and should the action be noticed immediately the pocketed C/M is returned to the board.