kdpierre:
Going from friends who were equals to still friends...but with one able to look at a situation and behavior and with just a wave of their finger, get their 'friend' to accept a genuine, unpleasant spanking without complaint or resentment.
This sounds quite a bit like your story "The Friend," which does detail some of the thought processes of a man who has agreed accept a close female friend as his personal disciplinarian (including being his 'keyholder').
I'm presuming that you're thinking of an account which explores the gradual but fairly consistent alteration in that type of once-equal relationship, until it's been transformed into a situation wherein one of them can simply say, "Get ready for punishment," and the other one will immediately reply, "Yes, ma'am" (or "Yes, sir").
The issue of how truly equitable the non-disciplinary parts of a relationship (marital/romantic, family or friendly) can be, when there is also a one-way punitive aspect involved, that's something I've cogitated a touch on myself...

--C.K.