Borrowed chords (modal mixture) are spelled using the parallel minor's flat-side scale degrees

Chords borrowed from the parallel minor (♭III, ♭VI, ♭VII, iv, ii°, etc.) are spelled with the flat-side letter/accidental consistent with the parallel minor's own diatonic spelling, not the major key's own accidental-preference side.

A borrowed ♭VI in C major has its root spelled G#4 instead of Ab4 -- pitch-class identical, but G# implies a raised-5th/chromatic-mediant reading rather than modal mixture.

Standard tonal-harmony convention.

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