Be Excellent To Each Other

iTunes on a windows machine.

iPhone 5 which syncs to the PC at home via said iTunes

That phone has never had music synced from the PC, and just has music bought on the phone.

I now want to add music from the PC. When I try to do so it says that it needs to wipe the iPhone to sync with the new library.

My question is this:

Does it totally wipe the iPhone, or just wipe the music from the iPhone? I can't ever foresee a reason why it would wipe everything, but that's what it seems to imply.

Can anyone give me a definitive answer before I destroy my wife's phone?

Is it the same PC that the iPhone Syncs to that you want to add the music from, or a different one?

Same one. Same sync as normal, just with added music. Apple help pages seem to indicate it just wipes the music. I hope.

And you haven't ReInstalled iTunes since it was last synced? If you haven't, I'd make a new backup, then go to C:\Users\USERNAME\AppData\Roaming\Apple Computer\MobileSync\Backup and make a copy of the newest folder to somewhere else (Incase iTunes decides to delete all Backups for no reason like it has done with my iPad before)
Then go ahead and see what happens. You can always restore from backup if it messes up.
Disclaimer: I take no responsibility if you lose data.

It's just the music library it'll wipe.