They are trying to brow beat him into being an informant. Their threat is the no fly list, which they were able to use without needing to go to court.
So why are people saying he should have asked for a lawyer? Sure, I guess, but that doesn’t help this situation at all. Why not suggest he should also eat a healthy breakfast before going to the FBI office? Yes, that is good advice in general, but doesn’t really matter for this situation.
The goal here was coercion via intimidation.
You are right, they can’t arrest him... which they didn’t try to do. They did something way worse, which was to put him on the no fly list and strand him out of country.
Avoiding being arrested was not the problem.
This isn’t an investigator trying to get information to bring to court, this is the FBI trying to coerce the guy into being an informant. They don’t care about his actual testimony at this point.
Instead he sent his family to lebannon and flew there himself.
Would have been cheaper just to lawyer up...
Lawyer should be able to help at least track down which agents did this, call bureau to confirm investigation and complain if things are sketchy, help avoid the no fly list or help get you off it... advice...