What would be the going rate for manual labor per hour, and what would say is a fair price for a friend? If it matters, the labor itself is not extreme (like laying bricks) if that should factor into your thinking. And, if the going rate is regional, this would be in the Mid-Atlantic area (but not upper crust, like parts of DC and Northern VA).
There’s no right answer, and working for friends/relatives can often lead to frustration and resentment. If the job isn’t too big, I try to do it for free.
Otherwise I try to set the price low enough that they understand they’re getting a deal, but high enough that I don’t fucking flip out if it ends up dragging on and on and on.
But again, there’s no right answer.
$10-15 an hour would sound right to me, unless they were working for less during their “regular 40.” Then I’d still pay them a premium over their “regular rate.” If it was close enough to the $10-15 range, I’d probably just go with that.
And I’d include the beer/food/other “refreshments.” 
Need more information on the nature of the work, and how long you expect it to take. Is this an actual job, or just work you’re doing to help out a friend?