Battery dies when it's completely shut off, or when "sleeping"?
If he powers it off completely, and the battery dies before the next day - either there is electrolysis at play - chewing up the innards of the phone. Thing is, this process does not last very long, and the phone will be completely dead after a couple days. (as in, it won't turn on)
Or (more likely) - the battery could be defective - and those can be replaced.
If he's sleeping the phone, and it's dying before the next day - poor cell service, having BT and Wifi turned on can run down a battery.
Is he using icloud? Another well known battery vampire.