I assume your friend who is good in airbrushing, does art work for PC Casings, Gundam models or maybe a Car Tuning art works .... and haven't done a paint job on guitar before.

Even if he's willing to airbrush your guitar for free or for a small amount of money, i would recommand you to send your guitar to a person who actually knows how to paint one.

You're dealling with wood here ... not plastic nor metal. If he makes a mistake, that mistake might in a way effect your guitar's sound, playabitily and its looks. Besides, he'll need to remove all the pickups, wires, tremolo and etc out from the guitar first. If he mix the paint too thick .... you'll have a hard time putting those equipment back.

Pros exist for a reason. Use them.

Just my 2 cents