To be honest... I won't volunteer to be their main photog if I haven't got experience not much knowledge in photography.
If it is a very close friend, ask him to hire a pro, and you tag along and take some pictures, enjoy yourself and give them some of the picture as "wedding gift". That take out stress on yourself while you enjoy being there on the day.
I don't advocate to do photography for free, but different mindset is needed when you are doing it just for fun.
As professional, we work in different level as it is something that put food on our table hence we understand how to do the sum on what to charge.
At the end of the day, have fun, learn photography and don't worry about the money side just yet.
I have a lot of pairs of scissors, I still go to hairdresser to cut my hair. Same thing as, if you have a lot of camera equipment, it doesn't mean, you should start charging people.
Being a professional is more then just knowing how to charge and knowing how to take nice photos.
Regards,
Hart