Yes, I have suggestions. Nine times out of ten, the reason for this behavior is that she has left every app on the phone running. Running apps eat up power, just how it is.
Here's what she should do. After logging in (she does have a password set, right?) press the home button twice. The screen will then show all running apps, and if you flick a given app upward, it will quit. You will find that everything you've ever run on the iPhone is still running, and the first time you do this, the list will be long. Quit everything that you don't need running overnight. Quit mail, and maps, and calculator, quit all that junk, and your battery will not get sucked dry overnight.
Chances are that there is only one or two apps that are responsible for this behavior. With some experimentation, you can figure out what the culprits are, and then you can use them when you need them and immediately quit them after you are done to save battery.
Lots of people blame the battery when this inevitably happens.