Prediction: If you don't do a clean installation your friend will pester the hell out of you because of all of the weird crap that will happen after the upgrade. You'll wind up wishing you had done the job right in the first place. But take care that you can reinstall all of the friend's favorite apps (or replacements for them) and take care of the data, too. Otherwise, you'll get blamed for missing apps and data.
Heck, come to think of it, you're gonna get blamed for anything that goes wrong whatever you do. No good deed goes unpunished.
Heh.