Re: Gibson LP selector switch
It sounds like one of the volume pots is de-coupled and the other is not. That's explains that behaviour.
When they are both coupled, they behave like Lew described, either one turned down turns down the whole guitar. The hot wire from the pickup is going to the middle lug of the volume pot and the output wire is on the outside lug of the volume pot.
When they are both de-coupled, turning one down while the other is up just shuts off that pickup. The hot wire from the pickup is going to the outside lug of the volume pot and the output wire is going to the middle lug of the volume pot.
It sounds like your friends guitar has one volume with the pickup hot going to the middle lug and the other pickup hot going to the outside lug. This is no big deal really, just a preferance.