That's for the actual voltage of the cells. An intelligent device like a phone will display relative (usable) percentage. If it didn't your battery would be dead (or catch fire) if you ever reached 0%.
They don't need to get down to zero for the battery to be harmed.
She gets a day or more out of it. Like it did when it was new.
So I don't agree.
When new an iPhone 4 would easily get at least two days. Lithium batteries also simply degrade with age, so it is highly unlikely her battery is still acting like new. And if it were I'm sure battery researchers would love to get their hands on it to figure out how she has a lithium battery that hasn't changed at all in five years.
It is not 1998 any more so don't stress.... IMHO
No, it isn't, that's why the weaknesses in modern batteries are different to those of back then.