That sounds much more like infidelity than whoring. The case against Diana going to Milohai is predicated on her prior commitment to Athena: absent that prior commitment there's no harm done.
Whoring doesn't imply infidelity; there is nothing in the definition of the word that requires that at least one participant be married. Excoriating Diana for whoring suggests that making the deal she did with Milohai was, in isolation from prior commitments, somehow un-virtuous or at least undignified and by analogy that prostitution is likewise undignified.