But I can’t find a way for my wife to edit on her iPhone. She can view ok, but she can’t add or edit, which kind of defeats the purpose of sharing it. She is a Hotmail/Live user and isn’t keen on setting up a new account just for one calendar.

Every guide I can find (that is up to date) talks about sharing to my own iOS device, which I have done, but there doesn’t seem to be anything about fully sharing a calendar to someone else, which in itself might be the answer.