I once knew of a girl who believed that the devil existed yet mocked any existence of God. I think she believed in angels as well.
What kind of questions or conversation would you have someone who holds these beliefs?
Maybe look to draw out who or what she believes the devil to be and why she believes what she does? If she's faced a lot of evil in her life it may be she concluded that some kind of evil entity exists but she saw precious little evidence of anything benign or loving. She may have accepted angels as the things that offered transient protection from the evil. (Obviously I'm guessing here, speculating as to what kind of life experience might lead to such a conclusion)
It's not a logically inconsistent position to believe that the devil exists but not believe that God exists, any more than it's illogical to believe that God exists but not the devil.
What is interesting is when people accept the idea of some kind of antichrist existing but reject the existence of the Christ. The very existence of "antichrist" implies the "Christ" because being the opposite of something that doesn't exist isn't entirely rational.
