You are right it is ambiguous but the best guess is that she started out as his patient, he became romantically involved with her and “treatment” went by the wayside. She got him to pay her all kinds of money and provided sex in return until she got tired of the old man, and found a lover her own age. But he couldn’t let go, hung on, became an albatross, as only a rejected lover who refuses to let go can be; she grew to despise him so together with her cronies she decided to dump him— permanently. Remember this woman was a Playboy model so she was used to the seamy underbelly of life.
Just a guess.
Reminds me of that awful movie, The blue angel.