have a dying alternator. If it's below 13-13.5 volts and she likely isn't charging enough to replace what she's using.
After driving around for awhile, park the car and wait 5 minutes. Test the voltage at the battery posts again. You should have 12.5 - 12.8 volts. Anything less than that and it's the alternator 99% of the time. If you have in that range but the next morning it won't start and the volts read much lower, you probably have parasitic draw from somewhere that is killing your battery.