Motorcycle charging systems have three phases which under normal circumstances, should be able to satisfy all of the power needs of the bike. You could almost look at it like three batteries in parallel. Now, if you lose one of the three batteries, the other two may be able to keep the voltage up on the bike at idle, supplying the headlight, ignition system, etc., but barely. Now, when you rev up the bike, the ignition system demands go way up and the two phases can no longer support the additional load and the output voltage sags. Another indication is sometimes the headlight dims in cadence with the signals at idle, or dims with the brake light.
Now, he may have a battery problem, but if so, it usually shows up at idle, too.
A good battery is essential for the charging system to function as it should, but with these symtoms as he presented them, I would be giving the old eyeball at the charging system.