Mr E was formally separated from his wife Ms A at the time. They are now divorced.
Ms A became involved in the treatment, and in all 29 sessions involved Mr S and Ms A, with Mr E only present for three of them.
Ms A said her first private meeting with Mr S was because she was worried about her husband, who had started smoking cannabis, around their children.
Mr E was going to join the meeting, but changed his mind.
In the following months Ms A and Mr S began to meet regularly, sometimes once a week.
The meetings were initially about Mr E, but the pair became closer and the meetings more intimate. They began kissing, and there was intimate touching.
The misconduct carried on over an eight-month period, all while Mr S was providing clinical services to Mr E.
After seeing emails between the pair, Mr E complained to the DHB he suspected his wife and former nurse were having a relationship.
When first told of the complaints Mr S denied there was any relationship with Ms A, and stated he had worked hard for some years with Mr E.
He said the investigation was "ridiculous" and that he was "shocked".
He initially said they had met to discuss Ms A's university work, but later admitted the intimate relationship.
At a meeting with the DHB, Mr E said he was very upset that through his involvement with mental health services his marriage had broken down.
He placed enormous trust in his relationship with Mr S, for example phoning him when he was feeling down.
He felt that Mr S saw Mr E at his most vulnerable and took advantage.
He was curious why Mr S and Ms A were going for "2-3 hour education sessions" at the beach, but he did not question it, because he trusted Mr S.
"Looking back now he feels manipulated and taken for a fool," the tribunal said.
The DHB's clinical team leader said she noticed a decline in Mr E's health during and after the investigation.
He refused support from mental health services, and no longer trusted mental health professionals.
"He went from being very trusting and open at the start to having a complete distrust of mental health services."
The tribunal found a "clear breach" of professional boundaries.
"It is not appropriate for a health practitioner to engage in an intimate relationship with a member of the patient's family even if the wife was estranged from her husband during that period.
"The couple were still in a close relationship to the extent the wife was involved in seeking treatment for her husband and attending counselling sessions to assist in his treatment.
"Mr S used his position to commence a relationship with his patient's estranged wife, at
a time when it should have been obvious this would risk harm to a vulnerable patient.
"The harm caused to the patient is a stark reminder of the reason for the professional boundaries set for nurses and all health professionals."
Mr S was suspended from practising for 12 months from the date of decision, on November 20 last year.
He also had to pay 25 per cent of the tribunal and Professional Conduct Committee's investigation costs, amounting to $7893.
