Ask your pard if it would be constitutional for the FBI to ask a foreign intelligence apparatus to surveil an American citizen.
But that's not the situation. Nobody asked a foreign intelligence apparatus to surveil an American citizen. Your scenario of outsourcing is not what happened. And my friend didn't say that Americans could outsource this so that foreign agencies spy on our citizens, over here. He merely said that if a foreign source with whom we have a treaty, using their own legal processes, in their own country, going after their own people there, then bump into evidence that an American committed a crime, they refer it to the FBI and it is admissible.
Here is what happened: the Spanish apparatus engaged independently in surveillance of the Russian, in Europe, talking to another Russian (apparently they were investigating the guy for possible money laundering in Spain). Apparently Trump Jr. was mentioned in the recorded conversation and the prosecutor turned the evidence into the FBI.
Are you under the impression that the Spanish prosecutor was surveiling Trump Jr? Again, that's not what happened, so, there is nothing unconstitutional about it.
What my lawyer friend said is that the evidence the Spaniards bumped into, can be turned into the FBI and it is admissible (just like the Interpol; these law enforcement agencies have treaties of cooperation and mutual admissibility) because it doesn't fall under the US Constitutional protections, given that it wasn't the doing of an American agency.
An European agency surveiled an European talking to another European. If evidence of a crime committed by an American then pops up in the conversation, the Spaniards notify the United States. Just like, if the FBI were (with the proper warrants) surveiling two Americans and they talked about evidence that a Spanish person committed a crime, the FBI would warn the Spaniards.
Got it now?
*Should* our protections extend to such situation? Maybe they should; I'm not disputing this part. But according to my friend, they currently don't. If you want your representatives to sponsor a constitutional amendment to include something about this, be my guest; but currently, there aren't constitutional protections in the US against evidence of a crime committed by an American, when it's discovered by a foreign agency operating abroad. The Spaniards reached their conclusion independently, using their own processes, supposedly did it legally too according to their laws, and they found something that compromises an American; given their treaties of international law enforcement cooperation, they warned the United States.
Now, let me tell you who my friend is. He majored in International Studies in college. Spent time studying in Switzerland and Italy during college. Law school in the Ivy League. Works for a major firm specializing in white collar crime. His firm does defend foreigners and they have offices in London and Milan (in addition to several offices in the United States).
Frankly, I'm quite sure he knows waaaaaaay more about this than you and me. I mean, I can speak for me. He knows way more than me, no doubt (which is why I asked him). I don't know if you are a lawyer specializing in white collar international crime (with a background in international studies and time spent studying in Europe), who graduated from one of the best law schools in the United States (and the world), and who works for a firm with international expertise. If you are, then, fine. If you aren't, chances are that he knows more about this than you do.
So, why not take the word of an expert?
According to him, if (and that's an if because we don't know what is in the tape) the Spanish prosecutors when surveiling two Russians in Europe, bumped into information that compromises Trump Jr., they can turn it into the FBI, and it is admissible.
The "if" is kind of likely, given what the Spanish prosecutor said: "The president's son should be concerned."
Why in the hell would he say that, if the content of the tape had nothing to do with Trump Jr.???