Your friend is partly right; Satan certainly does believe in God -- that is, he believes God exists, and he even knows that He is powerful. But the devil believes in God the way we might believe in one of our nation's enemies: we know he definitely exists, but he's still our enemy. The Bible says, "You believe that there is one God. Good! Even the demons believe that -- and shudder" (James 2:19).
Yes, everything the devil does is evil, but he also is a master of deception, and sometimes he's behind things that even appear to be good. Remember, Satan's one, all-consuming goal is to turn us away from God, and if he can do this by causing us to follow a false philosophy or mistaken religion or deceptive cult, he will gladly do it. The Bible warns that sometimes "Satan himself masquerades as an angel of light" (2 Corinthians 11:14).
Satan is powerful, and he will do everything he can to exploit our weaknesses and blind us to God's truth. This is one reason why we need to know God's truth as He has revealed it to us in the Bible, and walk closely to Him every day in prayer and fellowship with other believers.
But listen: Satan is also a defeated foe! By His death and resurrection, Christ defeated the powers of death and Hell and Satan, and someday His victory will be complete, when He comes again. Is Christ the foundation of your life?