Sorry about the brain hurt, but this can get even more complicated in that your guitarist could just decide to work with an A scale using no capo (or decide to play a C scale and capo on the 7th fret!) It is all the same key of A! You see how this complication can compound.

I think talking with the guitarist is the best idea. In the end that song is in the key of A, no matter how you get there. A few words to the guitarist would explain it all.

But then again, I never worked with a long distance musical partner. I have also never sold a single song and at this point it is likely that I never will. So take the value of my verbage with a grain operator ignorance.