I haven't yet read what the patient's condition was. See I know that in the rules it is not relevant but hear me out...
If the patient had a minor admission for something relatively innocuous, maybe re-setting a broken hand, laparoscopic hernia repair then MAYBE this could be a reasonable situation. I have met two female nurses who met their husbands in the line of their work, and nobody shopped them to the authorities. Their conduct remains professional and their relationships have evidently flourished. So surely this implies that it is possible to date a patient without having an inherent power imbalance that will lead only to misery for the patient. I would personally keep my mouth shut and pretend I don't know.
That said, if the patient was chronically ill, mentally ill or in some other way vulnerable, questions must be asked and Subcut did absolutely the right thing if this was the case. It is a very difficult situation to be in. As Subcut mentioned, NMC hearings are awash with mental health patients being shagged by male and female carers, whilst at their most vulnerable.
Of course the nurse's gender must be noted. Males are driven far more by sex than women generally and female colleagues opinions vary on this. I have known some suggest that all male nurses wish to shag their patients and others that recognise most are just the same as any other professionals (professional in their behaviour towards patients). However , as males are often viewed as the sexual 'aggressor', a male nurse having a illicit relationship with a female patient is far more likely to be questioned than a female doing the same thing, even in the absence of any obvious vulnerability from the patient. I have known no males admit to ever dating a patient of their relative. Although we males may well comment to each other about who we would and wouldn't; man chat style.
But in conclusion, the rules are clear and unambiguous (and there exclusively to protect the vulnerable). If the man was so stupid to get in this situation, I wouldn't lose any sleep about the consequences for him. Subcut, in the limited information you have provided I would say you did about the right thing.