He also says Nixon should return from Ireland, where he is understood to be scouting Irish talent, to face the accusations.
Nixon has become embroiled in controversy since he admitted to "inappropriate dealings" with the 17-year-old girl who last year released nude pictures of a number of St Kilda players.
"I spoke to Ricky Nixon on Saturday and had a brief conversation with him," Demetriou told a press conference on Thursday afternoon.
"The tenor of my conversation with Ricky was along the lines of how disappointed I was.
"He understood fully my disappointment and how I believed that [his] action was unacceptable.
"He admitted that he'd made a serious error of judgement, and, of course, I asked after his well-being. I don't know the full [story], I don't think many of us do."
The AFLPA's Agent Accreditation Board has been examining Nixon's conduct and will hand down a preliminary finding on Thursday afternoon.
"The AFLPA are at the moment quite rightly undertaking their investigation," Demetriou said.
"That may take even beyond today and I think all of use should allow that process to run its course, because then all of us will have more information before we start passing certain judgements."
Nixon recently flew to Ireland where is believed to be involved in a recruiting program to bring young Irish athletes to Australia.
Asked if Nixon should continue in that role, Demetriou said: "No. I think at the moment Ricky Nixon fully understands that he needs to face his issues here and he, as I understand it, has agreed to participate in the process involving the AFLPA.
"I think until that process is complete that I don't think he should be continuing what he's been doing in the past."
However, Demetriou was at pains to point out that Nixon's behaviour should not be allowed to tarnish the substantial amount of work the League had done as part of its respect and responsibility policy.
"My concern and the AFL's concern is not for Ricky Nixon," he said.
"Our concern is, today, as it's always been, for the welfare of this girl.
"This story is not about Ricky Nixon. This is a story about a girl, a child, and we are concerned about her welfare.
"You can dress it up any way you want, and you can sensationalise it any way you want, and you can make this about Ricky Nixon being a player agent, but our concern, first and foremost, is for the welfare of this young girl."
