If he has a bad battery that keeps the charging system below the regulator's set point, the regulator will keep full current flow to the rotor windings.
Normally the rotor will have 2 to 2.2 amps for the first 10 to 20 minutes of a ride to recharge the battery. Once the battery is fully recharged, the regulator reduces the rotor current to around 1 amp.
If he was trying to charge a bad battery, the rotor current would never reduce, but stay at the 2 amps.
Power (watts) = current squared X resistance
For a fully charged battery: 1 squared X 5 = 5 watts
For a battery that is bad and does not charge: 2 squared X 5 or 4 X 5 = 20 watts
So, his rotor may have produced 20 watts of heat for the entire trip, which is 4 times the normal 5 watts.
Double the current = 4 times the amount of heat
What type of battery does DESTROY have and how old is it?
DESTROY should charge his battery with a bench charger for say 10 hours, and see if it ever reaches a full charge voltage, and then have it load tested.