Just because their social and interpersonal interactions occurred only online doesn't decrease the strength or potential strength of their relationships. In fact, some research has shown that social interactions and relationships can often be stronger online, because of the disinhibitory effect of online communication. The professional therapist who knew Mr. Froistad decided that, based upon his own personal observations and interactions with Mr. Froistad in the past, what he was saying was highly unlikely to be true. That's just untenable. I have been on mailing lists where the indignant professionals on the list have said they called the therapist's professional association to file an ethics complaint.
Privilege ends where public peril begins.
While some states have adopted this as law, others have not. It is, however, considered standard practice within certain professions and failure to act could result in professional disciplinary action brought against the therapist. Well, in the latter question, the harm has already been done. But this is not at all clear-cut. In this case, nobody was aware of Froistad actually committing the crime.