Our team supports the antics of about 75 Iphone and IPad users. We've seen everything from tired batteries to broken screens. Usually the user expects a new phone. ("my Iphone 4 is broken, can I have a 4S now?")
They always go perfectly (at least the second time!) actually they generally go well the 1st time as the tools do the job and the online guides are done so well.
Watch the video's a few times before you try the 1st repair. Some things take a little harder prying or pulling than you might expect. Don't worry, it's already broken right?
Set good rules and expectations before you start. Make sure you don't accept responsibility for lost data, get the user to back things up before you work on their device.
Be Brave and Good Luck