Woooah there
Yes - that will fix the problem if it's software related for sure, but, there are some other things that may be worth trying first which won't require the poor guy to have to re-enter all his settings, etc.
He implied that he just installed an app. There are a number of apps that cause this very problem if they run at startup. Many users don't realise there's a problem until they reboot their phone. There is also a known bug in Android 1.6 where if an app tries to access data while the WiFi service is initiating (during powering up of the phone) it fatally crashes Android core. When this happens the screen remains black after the G1 logo for AGES (10 minutes in some cases), but Android does eventually boot once it's finished crashing several times.
The newly installed app could well be to blame.
As such, there are two ways around this that are worth trying before a full wipe.
Try booting in Safe Mode.... you do this by holding down Menu while you turn the phone on, and keep holding Menu down until it boots. This stops any app running on startup. You can then uninstall it and reboot and see if it was actually that app causing it.
If it's the WiFi bug I linked above, simply waiting say 15 minutes might result in the phone booting.
If that fails, wipe it as Simms22 said.
If that fails, AND your friend has installed a custom firmware on the phone (he would know if he had), then you could try flashing a different/newer version of the firmware. Note that flashing the same version won't help unless something went wrong with the original flashing, which would have almost certainly been detected at the time - it's the wiping that is required when flashing a customer firmware that actually fixes most problems.
If you get here, then it's a hardware issue and the phone will need repairing.
