The theory of borrowed chords is that there is a set of chords that are diatonic to the major and also the minor with the same root. Thus, diatonically, C major, for example, contains C, F and G maj., D, E and A minor and B°. Since the parallel key is C minor, the diatonic chords from that are C, F and G minor, E♭, A♭ and B♭ major and D°.
Using the relative minor of C major - A minor - throws up a few other chords, due to the vagaries of minor diatonics. The basics will be the same as C major, but since A naturalmelodic minor contains F♯ and G♯, other chords are available - D major and E major being the most relevant.
I guess at that point, the parallel of A minor is relevant - A major. So we then get presented with A, D and E major (not much variance there), F♯, C♯ and G♯ minors, and B°.
Which possibly goes a long way to explaining why so many different harmonies can and do work considering one specific key... Probably easier to say 'these few chords don't work too well...x, y , z...
Funny that the diminished are basically the same, or not so, considering there are only 3 to choose from!
