Technically, that’s a vulgar Canadian film.
If the friend is at all interested in technical stuff, I’d recommend WarGames. It’ll give some dimension to the Cold War fears that many Americans over 30 still remember.
Exactly. I didn’t offer the movie, I simply explained why someone may see it as such.
I would like to offer the Peanuts specials from the 60s. Made for TV. NOT the later ones, only the ones from the 60s. Something about that Charlie Brown fellow and all those around him. Especially the Christmas special.
Plus that incredible music! VGT’s Linus and Lucy track is stuck in many people’s minds forever since seeing the Peanuts gang on TV.
Or at the cinema.
Or on reruns in the 70s
Or on VHS in the 80s
Or on DVD in the 90s
Or on Hulu or whatever you kids are using now.
Back to the Future. The first one, at least. Harold and Kumar Go to White Castle. My Cousin Vinny. The Karate Kid. The Rocky movies, at least the first three. Smokey and the Bandit. Several of the Star Trek movies. At least II, III, IV, and IV, Generations, First Contact, and the recent one by JJ Abrams. Platoon. Full Metal Jacket. Wall Street. Butch Cassidy and the Sundance Kid.
William Shakespeare is very much a part of America. Not “American Literature” but American nonetheless. Shakespeare wrote plays, and play are (IMHO) primarily about being performed and the works of William Shakespeare have been performed countless times in the United States and were often quite popular.
I don’t really think that the plays of Shakespeare are a paragon of what the OP was looking for–or even really what the OP was looking for at all, but I do think that there’s a case to be made for Shakespeare as an important part of the American experience.
Death Wish
The Magnificent Seven
Saving Private Ryan
The Big Chill
Billy Jack
Walking Tall (1973)
Rambo
The Man Who Shot Liberty Valance. Big stars whom your friend must know about, politics, the importance of the noble lie and the little guy standing up.
I could see an argument made that a foreign film about America could be considered part of the American canon. I suppose it depends on how you define “American canon.” If you approach it as, “Major works created primarily by Americans, for American audiences,” (which, I think, would be the traditional definition) then Monty Python is right out. It’s no more than 1/6th American at best, and was never conceived as something that would appeal to American audiences.
On the other hand, if you define it as, “Major works that are about America, American ideals, or describe the experience of being in a particular time and place in American history,” then Monty Python is still out, because there’s almost nothing in Monty Python that’s “about” America. It was popular in America. It influenced American comedy. It had an American in it. But it was about the United Kingdom.
Or, I suppose, you could just describe it as, “Stuff that’s popular in America.” But that’s not nearly as interesting a question, IMO.
Raiders of the Lost Ark
Fight Club
Lonesome Dove
Batman (1989)
Revenge of the Nerds
Die Hard
Aliens
A Christmas Story
FTR, I like the suggestion of Monty Python and the Holy Grail, as it is key to a subset of Americana of which many of us are a part. OTOH, I don’t think that it defines Americans as a group - many people just haven’t been exposed to it. So while I would include it as a possibility (and hence, not ban foreign films per se), I would probably not include it in a final list. I also think that MP & THG will be about as close as a foreign American films would come.
What I’m looking for are films that are essential to the American experience, so my title could be better. I wouldn’t be surprised if MP & THG was more popular in the USA than England (although this is said without a clue as to the data). The Mad Max movies might be another example, especially the later ones that became less Australian.
American Pie
The American President
American Beauty
Let’s check the OP (and not get pedantically focused on the title):
Your question may be more interesting, but it isn’t what he’s asking. He wants to know what films an American would find it important to have watched. And, to this day, a lot of Americans reference their humor, as if they expect you to be familiar with the show. That means it’s important.
It has nothing to do with popularity–it has to do with what movies you think people SHOULD see. Something foreign that becomes that popular is probably important to see, even if just to help you figure out what Americans appreciate about the foreign work.
Lot of good choices; much has already been covered. Except for the horror genre. At the very least give him a foundation:
Halloween (the original)
Friday the 13th (the original)
Nightmare on Elm Street (the original)
Poltergeist
An American Werewolf in London
Regarding the Monty Python discussion, I’ve heard it said that Life of Brian was England’s favorite while Holy Grail was America’s, so in that respect Holy Grail is a pretty good choice.
The Maltese Falcon—he must learn of Film Noir, to be a Real Amurrican.
The Big Lebowski.
Or any of the Coen brothers movies…Fargo, O Brother, Where Art Thou.
It was (half) directed by an American… then again, I think of most of Hitchcock’s movies as American. 
I think some Will Smith films, especially Bad Boys or the like, more Tom Cruise, like Gerry McGuire or A Few Good Men, Saturday Night Fever, Grease, James Dean, Brando, Beatty, some musicals (Anchors Aweigh? Guys and Dolls? West Side Story? Paint Your Wagon? I’m not sure…).
Boyz 'n tha Hood, Summer of Sam, Do the Right Thing.
Porkys. Animal House. The first two National Lampoon movies. Mel Brooks, esp. Young Frankenstein.
The American take on martial arts films, like Best of the Best, Karate Kid, The Last Dragon
Cannonball Run. Smokey & The Bandit. Every Which Way But Loose. Dirty Harry. Firefox.
Disney movies/TV that aren’t cartoons, like Blackbeard’s Ghost, The Love Bug, 20 000 Leagues, Davy Crockett, Darby O’Gill, Old Yeller
The Apartment: sex, corporate greed, and the little guy.
Scarface. I’d go with the original, but I’m like that.
Broadcast News
Wall Street: Yuppies suck
The Big Chill: Yuppies don’t suck
Mr. Mom: Unemployment makes you adjust
Tootsie: Unemployment makes you really adjust
American Graffiti
Beach Blanket Bingo or another beach movie
To Kill a Mockingbird
ETA: Wall Street and American Graffiti were already mentioned. Sorry.
Top Gun. Brian’s Song. Death Wish I. The Untouchables. The Dirty Dozen. Old Yeller. Pulp Fiction. Glengarry Glen Ross. Grumpy Old Men. The Blues Brothers. Happy Gilmore. The Godfather I and II only.
BTW, I checked and this fellow is 32 years old, so I’ll probably skip some of the older stuff and focus on the types of things a similar 32 year old American would have experienced, to the extent I can do that given my own generational bias.
I would argue that those are both part of the American canon. Most Americans have seen the MP movies and most Americans were forced to study/bef amiliar with the works of Shakespeare. At the very least he would need a passing familiarity with Hamlet, MacBeth and Romeo and Juliet.