I do not see any way this is covered by the various sexual harassment rules or laws with regard to the power relationship between a boss and a subordinate.
This particular situation isn't about harassment. Harassment occurs when one party is trying repeatedly to
establish a sexual, or at the very least a sexualized, relationship with the other despite explicit rebuffing. In this case the relationship between the two primary actors began with mutual consent; my point is that that consent in itself doesn't automatically make it ethical.
For one thing, as noted in the original story, Hill was already committed and did not initially have any permission at all to seek outside activity, and enabling someone else's infidelity is still unethical, if not quite as bad as committing it oneself. For another, while sexual and economic relationships
can coexist ethically between the same people, it generally requires explicit and formal commitments to achieve; when the topic deliberately goes unaddressed, it creates ambiguities and uncertainties that, even if not consciously exploited, can nonetheless undermine the relationship. (Consider, if nothing else, that asking someone about money he owes you can be a
lot more fraught if you're already sleeping with him.)
The simple rules "Don't sleep with someone already involved with someone else", "Don't sleep with someone who owes you money unless you absolutely don't care about getting the money", and "Don't sleep with someone who isn't free and willing to commit their life to being your exclusive partner" would have prevented all the chaos outlined in the original post. That basic chastity covers all this is no coincidence.