I have a friend that believes that God uses the devil to kill Christians when it's their time to come to heaven. I told her no, God doesn't need to use the devil. Please clarify this for me and give me scriptures to prove it to her. Thank you!

Yes and no. Satan does have power over death. It’s part of his dark empire. (Hebrews 2:14) 

God permitted the devil to be present in the Garden of Eden. Satan was the deceiver, who tempted Eve and indirectly Adam to sin. Their sin brought death upon us all. “…in Adam all die…” 1 Corinthians 15:22. “By one man’s offense death reigned,” Romans 5:17. Death is inescapable. Satan knows this. So in a broad way God allows Satan to kill everyone because it is not what God originally intended when he created man. 

Satan did attempt to kill our Lord. He used Judas as his willing instrument. John 13:3, “…the devil having now put into the heart of Judas Iscariot, Simon’s son, to betray him.” The betrayal was the spark that set in motion Jesus’ arrest, trial, conviction and death. 

But God is greater than Satan and nothing could happen to Jesus unless it was permitted by the Father. Jesus told Pilate, “you could have no power at all against me except it were given you from above,” John 19:11.  

Jesus had to die to be the Redeemer of mankind. Jesus willingly cooperated with God. “I lay down my life…No man takes it from me but I lay it down of myself,” John 10:16, 17. 

Jesus knew His life was protected by God. (Psalm 91:11) The same would be true of all Jesus’ followers. True Christians have angels protecting them. “The angel of the Lord encamps around about them that fear him and delivers them,” Psalm 34:7. No spiritual force (including Satan) can separate us from the love of God. “For I am persuaded, that neither death, nor life, nor angels [evil angels] , nor principalities, nor powers [of darkness]  … shall be able to separate us from the love of God, which is in Christ Jesus our Lord,” Romans 8:38, 39. 

Satan, primarily, wants to destroy our faith. “Be sober, be vigilant; because your adversary the devil, as a roaring lion, walks about, seeking whom he may devour,” 1 Peter 5:8. Jesus sternly warned Simon Peter of this same danger. Luke 22:31, 32 (NASB), “Simon, Simon, behold, Satan has demanded permission to sift you like wheat, but I have prayed for you, that your faith may not fail…” 

We do not completely understand spiritual forces working beyond our physical world. What we can be certain of from the scriptures is that true Christians are in God’s hands. Nothing happens to them accidentally. Especially the manner and time of their death.  “Precious in the sight of the Lord is the death of his saints,” Psalm 116:15.