Playing chess using a chess engine

I think that one of my chess opponents is using a chess engine when he plays online chess. The chess engine is always connected to www.chess.com I know that because he always  responds to my moves within few minutes after I am making a move no matter if it is late at night or early in the morning. Do you know if this is against the rules of www.chess.com ? I believe it is not fair someone to play against an engine except if he knows that the opponent is a computer. What is your opinion ?

Here's waht I believe...

http://support.chess.com/Knowledgebase/Article/View/124/0/cheaters--cheating-what-you-need-to-know

Thanks Jion_Wansu but your reply has nothing to do with my question

It is patetich

He could be playing on his cellphone.

ppl using cheats spoil themselves and in the long run they will mess with their gameplay and be exposed. Just ignore those nerds. Also gg and ty.