The "Holy" Quran teach that Jesus was just the prophet like Moses, etc. They do not believe that salvation comes by Christ. I actually had a discussion with my friend and found that he's a Muslim and so I tried to help him but he's so deeply indoctrinated by it that it's hard to convince him.
Dear Brother @Marek I hope some of the links associated with the questions below can help you with/in answering your friend and his questions/rebuttals. However, the powerful weapon you have is prayer. Bring him before the throne in heaven in supplication and you will be heard! Joining you in prayer for your friend. With the Love of Christ Jesus. YBIC Nick <><