Are you going to be the only photographer? Are you being paid? What does your friend expect you to capture and deliver? Is he expecting pro-grade photography? These are important things to clear up with your friend.
It is also good to let your friend know that you are relatively new to photography and to not expect too much from you, or demand too much from you. If your friend is gonna depend on you to capture all the vital moments, make it clear to your friend about your capabilities, what you can and cannot achieve, and that if he wants all the memories captured safe and well, to hire a pro.
No offence to you, but weddings are one-offs and cannot be redone if the moment is missed.