When he uses a charger that limits the voltage to the float voltage (13.7V at room temperature), he can leave the charger always connected. Of course the power consumption of the charger itself (during night time) should be low to avoild discharging of the battery.
When using some types of chargers with "boost" function, he may run into problems. When the motor starts, the charger may think that the battery is in low SOC and that may start a boost cycle. Depending on the implementation of the boost cycle, battery life may reduce.