This is an excellent question. You cannot blame your friend for bringing this up. One thing to be aware of, the idea of the trinity, is, in the end, not completely logical by human thinking. Just look at John 1:1. John says that in the beginning was the Word. The Word was with God and the Word was God. How can someone be with another person and be that person at the same time? From a human perspective, this is not possible. If you expect to give your friend a closed, logical explanation, you will be disappointed. What I say is that you should present to your friend what the Bible says. Given that Jesus raised from the dead, that he was sinless, that he fed the 5000, that he turned water into wine, that he raised Lazarus and that he fulfilled all the prophecies of the Messiah (Luke 24:44) I think we have no choice but to listen to him. Jesus said things which imply that he is ONE with the Father, yet he talked to the Father. This is not totally logical, but this is what Jesus said, and he has every right to tell us what is true. In fact, he came to us full of grace and truth (John 1:17). Jesus said "I and the Father are one." (John 10:30) When he said this, they picked up stones to stone him, because he had obviously claimed to be God. Yet he said, "For I did not speak of my own accord, but the Father who sent me commanded me what to say and how to say it." Jesus clearly said that his Father spoke to him, yet he said that he and the Father are one. I do not believe you can make this apparent contradiction go away. You and I need to let Jesus speak for himself. He certainly deserves this because of the life he lived. Jesus is with God and he is God! Jesus is our God and Savior (Titus 2:13). I hope this helps.
John Oakes, PhD