Trust your gut, brestmilklatte. It does not look good from what you have posted, imho.
I feel that if he is maintaining the connection with her as some sort of extension of ethical responsibility, then he could have, should have given you a heads up to the "overtime" that would be necessary on the ex-patient's behalf (which can be discussed without the personal details being violated). But he did not do this; red flag.
Further, thinking on "ex-patient", it seems she would seek professional help from someone else at some point (not years and years down the line)...making contact with him a conflict of interest. So this smells a little fishy. Does he "treat" her for free, and why do that for so long? Does he maintain any other "ex-patients"? I will guess not.
Going with chubfud's line of thinking, my prediction is that, at the beginning he told you "ex-patient", when perhaps she is/was really "ex-partner", and that shut you up good and proper.
I hope I'm wrong, of course. I rescanned the thread and could not find it so sorry if asking you to repeat, how long have you been with/married to your DH?