If this is a serious project, you would be wise to follow Alcove's advice. If you do retain an attorney, make certain s/he specializes in the entertainment industry.
If this is strictly a low-budget spec project, you obviously can charge whatever you feel your time is worth. You mentioned it's for a friend, so you might be inclined to dispense with formalities, but if nothing else, have a contract drawn up.
You can also look at the current WGA minimums for ballpark rates received by the pros.