Back again folks, to say "Thanks" for all the wonderful input. Just
to clarify, I am a folkie who plays guitar and I have three upper-end
Takamine's and an upper-end Breedlove; but, it doesn't seem to matter which one I use - the consternation remains.

Don Firth - you may be right about the "one-upmanship" thing, that
could be happening.

Jeremiah - you may be correct here that she may be "cursed" with
perfect pitch; and, though my ears are certainly not as "gifted" as
hers, I do have quality electronic tuners to keep me in line, and,
I do sing on key, rarely get off, and beat myself half to death if I do.

By the way folks, she's a very nice lady, plays great, sings just
beautifully and,...and,...well,...doesn't like my tuning. Cheers.