Leaving the Beo5 on the table charger when not in use is not a problem and it has been designed to do just that.
It is certainly not unsafe leaving it on the charger, in fact the Beo5 software actually monitors the charging of the battery and when the battery is fully charged there will be no current to speak of flowing.
If you wife is not concern about leaving your computer switched on, then she certainly should not need to be concerned about the Beo5