In Matthew 16:18, Jesus responded to Peter's revelatory confession of Christ by saying, 'Upon this rock I will build My church.' Was the rock Peter, was it the revelation of who Jesis is, or was it something else? We believe that the rock was neither specifically Peter nor specifically the revelation of Jesus. We believe it is best understood as the relationship that was formed between Peter and Jesus based on their mutual recognition of each other. Basically, Peter says, 'I know who you are,' and Jesus responds, 'I know who you are.' This mutual rcognition formed the basis for a genuine relationship between them. So what we are saying is, the church that Jesus is building rests upon the personal relationship between God and man.
In Galatians 4:9 Paul wrote, 'But now after you have known God, or rather are known by God, how is it that you turn again to the weak and beggarly elements....' What he was saying was, 'How could you go back to the powerless Jewish law now that you have entered into a relationship with God based on the knowledge of who He is...and His knowledge of who you are?' Paul was willing to lose all that he had if that was what it took to gain the 'excellency of the knowledge of Christ Jesus' (Philippians 3:8). To Paul, this relationship with Jesus was worth everything.
Your Catholic friend needs to know that there is no biblical evidence that Peter ever went to Rome, much less that he was the first pope. He also needs to know that a real relationship with God is possible for him, but he must be willing to give up his Catholic rituals.