It depends on the specifics. Your friend is basically accepting not only the word of the guy taken into custody completely at his word but also taking his and the media's characterization of it totally at face value. I don't care if he's a former Supreme Court clerk. It's woefully sloppy to just jump to that conclusion based on the facts and evidence that we know and in light of what we don't know.
And of course, this isn't a legal explanation at all. It's a political one, and it's a shallow one at that. First, even accepting the guy's word at face value, fascists don't usually arrest people and then release them. When the Gestapo arrested people, they ended up in Dachau or Sachsenhausen. They weren't let out a few minutes or hours later.
Second, why call it fascistic? Of course, communists and authoritarians of all kinds do what fascists do. In fact, China is doing it right now to hundreds of thousands of people who aren't rioting.
Third, was it really violent? Did they beat his *** unprovoked? Did they shoot him? Doesn't look like it.
Fourth, what looks far more fascistic is the actions of Portland officials and the "protestors." We have people destroying private property and inflicting bodily injuries to intimidate and force their will. Kinda sounds like the Nazi party SA. Then we have local officials telling the legitimate police not to enforce the laws against them. Kinda sounds like what Nazi and Nazi-sympathetic government officials did.
Here's the bottom line on the federal officers coming in. If they're defending federal property or arresting people reasonably suspected of damaging or threatening to damage federal property or harming federal employees or doing anything to stop or inhibit federal employees from doing their jobs, they have every right to be there. Those are violations of federal laws, and they can be enforced by federal law enforcement. The mayor and governor have absolutely no justification to complain about it at all. They don't have to limit themselves to staying near the federal property either. They aren't security guards. They're law enforcement officers. They can chase people down just like any other cop.
They don't have to use marked vehicles. These officers probably don't have cop cars. They don't usually need anything like that. They are probably using general government vehicles, or they are on temporary duty assignments using rental vehicles.
They can release people if they choose. If there is reasonable suspicion of the person committing a federal crime, they can confront and detain the person briefly, and choose to release him. That's not a violation of the 4th Amendment.
Of course, there is a line at which this becomes inappropriate, and if you can find evidence of it (not media speculation), I'll condemn it. Where is that line? It is when federal law enforcement officers stop and detain without reasonable suspicion or arrest without probable cause. It is when federal law enforcement start acting outside their jurisdiction. For example, can they go arrest the people who burned down the Portland police union building? No. That's arson committed against non-federal property. It would be a state crime. If the mayor and governor are ok with this sort of thing, they can choose not to enforce their own laws if the people tolerate it as you seem to.
Why don't you see this happening more often? Because it's usually not necessary. Most of the time, local officials don't tell their police to basically shutdown and let angry mobs form and destroy property without recourse. Under normal conditions, someone caught vandalizing a federal building would get caught by a local cop and charged. He might get turned over to federal authorities for prosecution or might get prosecuted at the state level (since it's also a violation of state law), but you wouldn't need federal cops to apprehend him.
However, we're see a reemergence of large scale politically protected criminal activity in some areas. If a crime is being committed for a political reason local officials support, those officials are choosing not to enforce the laws against them. It's a reemergence, because of course, we saw Southern officials do the same thing when they let the Klan commit crimes against blacks with no recourse. They feared the Klan's political power more than they feared the crime itself. When that sort of things happens, you can't have federal law enforcement enforcing state law, but you can have them fill in the gaps of enforcing federal laws that the local police used to assist in enforcing.
