charging ?
 

buddys bike , ac volts out of stator is 58 v. battery has been going dead while riding,battery voltage at battery while off is 12.9 ,fire it up battery voltage is 12.3 , is this a bad voltage reg? thats what i am thinking, the drop is probably from lights and starting.

It very well could be a voltage regulator but to make sure put your volt meter on the battery and crank the engine. As you said, the voltage should drop while starting and then stabilize once started. Now raise the RPMs to around 3K and watch the volt meter, it should climb to at least 13.5 - 14 and shut the throttle.
Even if the voltage does climb, it could still be the regulator working intermittently. Make sure your battery connections are tight (10mm wrench), that is probably 75% of the problems with the charging system I see. If your connections are tight and the battery is still going dead, before buying a voltage regulator ask the shop to load test your battery ( shouldn't be any charge if you're there to buy a $120 battery/ voltage reg) to make sure it isn't the culprit.

Good luck.

Before you spend any serious money/time on a VR, load test your battery. A bad battery will often hold voltage at "normal" levels until it is placed under load.

the battery is brand new ,about 2 weeks, its a buddy of mines bike, when i rev it last night there was no increase at the battery,just battery voltage .thanks

Then it is the VR, good luck.

The older carb bikes (2002) at idle will have about 13.1 vdc and above 2000 rpm 14.1 14.4 vdc. The EFI bikes should be 14.2 vdc at idle and up. I've got a little volt meter from Kurakyn, LEDS show the voltage. I also go with the VR.