i suggest you ask him how he cleared 15 minute melee, then make him show you his strategy...then you can laugh as the liar fails after 5 mins...and say 'so how did you unlock falco then???'. assuming he's the noob we all know he is, he wont know anything about playing for a certian amount of matches to unlock characters, so he wont be able to say anthing. if he does try to make any excuses about failing, be sure to demandingly say 'NO JOHNS' at every single one.
if he somehow does clear the 15 min melee with the strategy he says, then...you should check his 'bonus records', and see if he has any rediculous ones like 'youve played 1,000,000 matches' or anything. that right there is screaming AR. unless your friend is mew2king, in which case i wouldnt doubt he could unlock everything in a few hours...
third way to discover the truth is wait till he's not at home, then break into his house and search his room for the AR!!!! although im not sure thats legal, but im sure the judge would understand. maybe. actaully, he'd probably say 'NO JOHNS!!'