If your friend is a protestant, you may want to use these scriptures:
1 John 4:1-3:
*Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.
*
The Church-approved Marian apparitions certainly pass muster here. Our Lady always points to Jesus Christ as Lord. I think this is consistent with the scripture’s description of a spirit from God.
Matthew 12:22-27
Then they brought him a demon-possessed man who was blind and mute, and Jesus healed him, so that he could both talk and see. All the people were astonished and said, “Could this be the Son of David?” But when the Pharisees heard this, they said, “It is only by Beelzebub, the prince of demons, that this fellow drives out demons.” Jesus knew their thoughts and said to them, "Every kingdom divided against itself will be ruined, and every city or household divided against itself will not stand. If Satan drives out Satan, he is divided against himself. How then can his kingdom stand? And if I drive out demons by Beelzebub, by whom do your people drive them out?
In the passage from Matthew, Jesus points out the devil would certainly not drive out his own demons.
In the Marian apparitions, I think we see a similar logic. Our Lady always points the way to Jesus Christ as the Son of God; this is inconsistent with your friend’s claim about the devil’s work. If the Marian apparitions are the work of the devil, why do they all point to Jesus Christ as Lord? I think your friend’s argument breaks down here as well.
Hope this helps, peace.