"Two friends of little to moderate musical experience, one singing and one playing the guitar, want to work out an arrangement of a traditional song. The guitarist cannot tell which key the singer is in (or, what note she is singing) and vice versa. No other instruments or (external) tuners are available, and so their musical efforts are frustrated. Unless, of course, someone "blessed" with an "internal tuner" (the knowledge/ability to tell the guitarist which note/key the singer is on/in), happens to be in the room."

The scenario you are describing, daylia, has little or nothing to do with perfect pitch. If a singer or a player can't tell what note - as found on the instrument- s/he is on, some basic education is called for. If the ppp is there to tell them what note it is, they have learned nothing for the next time.

Try a different scenario! ?:~)