I'd start by figuring out how many hours I was going to use to do it. And remember to calculate time for the layout/sketch you do before putting it on the wall - and add some time for "troubles" that delays the project. Then when you've got a ball park figure - double it, and you've probably hit the mark
Then you think on how much/little you will charge per hour to be satisfied - and feel free to tell him that this is the FRIEND price and not what you will charge for other peoples walls
and congrats on the commission and good luck