I can't really give a full explanation of everything your friend needs to learn, so I'm pretty much gonna list things and he'll have to look them up more himself.
In no particular order, your friend needs to learn to:
fastfall, lcancel, shorthop, wavedash (i saw him wd once or twice but he really didn't have it integrated into his game), dashdance (you don't get to do this against projectile-spammy characters like samus as much as you do against other characters, but at close range it's necessary in some situations), short hop double fair (I find shorthop fair to uair is often useful against samus too), swipe missles with ftilt, use utilt, use dtilt (often after a shffled fair or to edgeguard), edgehop bair (to edgeguard), edgehop fair/nair/uair (to return to the stage), space his aerials correctly (usually just hitting with the tip of the sword -- 3:10 is a good example of a poorly spaced aerial and what happens when marth spaces his aerials poorly), tech, ledgetech, use sidestepping, stand further from the edge when edgeguarding with fsmash (far enough so the tip of the sword barely extends past the edge), probably some other stuff I'm forgetting.
Just as important, he fell into the common trap of newer players where he basically just tries to hit you with his sword -- no looking for openings, no trying to draw a whiff, no variation of styles to keep his opp off-balance, no trickery of any sort. He needs to be willing to play patiently and wait for openings/create openings. Good marths constantly pressure their opponents (sometimes by abusing marth's range, but often just by staying at striking distance -- fsmash tipper range) but are very careful about when they attack and what moves they choose to attack with. Have him watch some pro videos (looking for the elements I mentioned he needs to work on) and browse this site at length.
Best of luck.
