As you are probably aware, the TRACON does not actually violate anyone. It's not their job. When a loss of IFR separation occurs, a report of the incident may be required by policy under a number of circumstances. Being asked to contact the facility by phone sometimes results in a discretionary judgement by the person on the other end of the phone that the matter need go no further. But where policy demands otherwise, any information gathered from the pilot during this phone conversation may become a part of that report. If that report is forwarded to the FSDO, a safety inspector will be assigned to investigate the incident. That investigation may include reviewing RADAR data, ATC voice recordings and interviewing the controller(s). And of course the pilots involved. Contact with an inspector is usually the first indication that a possible enforcement investigation is under way. Anything said to the inspector may be used as evidence in any enforcement action.
Qualified aviation legal consultation should be sought by these pilots at the earliest opportunity. The legal advice obtained may prevent the situation from becoming worse than it has to be. Beyond that, I can only say that any legal advice should come from a qualified professional specifically experienced in the area of pilot enforcement actions. There is no reason to believe that the company's interests are coincident with the pilot's interests, so be careful about the creation of company documents related to this incident before having that consult.
Best of wishes to your friend. This kind of thing could happen to anybody. Only a moment of distraction...
Best,
