Have you ever wanted to say “I don't get it” without making yourself look stupid? Shakespeare is your solution. Firstly, if your friend knows Shakespeare, he would think you're cool. If he's not Shakespeare-literate, you still sound great. Well, here's some help.
“I understand the fury in your words, but not your words.” - Desdemona, Othello
Othello was full of wrath, and Desdemona innocently said this like, “Calm down, Honey. I don't know what you're talking about.” 'Fury' means super ARRGHHHH! type of anger, not the Greek furies, though the word was probably derived from that. But hey, why not calling your frienemy a Fury anyway? It's like, “Hey Bro, you act like a monster and I still don't understand any word you say.”
“More matter with less art.” - Gertrude, Hamlet
Polonius was going down the rabbit hole explaining Hamlet and Ophelia's love story in superfluous lines. The Queen wanted it quick, so, “Stop all the fuss and say what you mean.” CAUTION: Don't say this to artists. It would be highly offensive.
“It was Greek to me.” - Casca, Julius Caesar
Do you understand Greek? If you nod, then this is not for you. Cassius and Brutus wanted to know what Cicero said in the Lupercal, and Casca McCoy answered, “I don't speak Klingon.” No wonder Uhura and Spock said behind his back, “He was smarter at school.” (I'll leave the identity of Uhura and Spock to your own imagination. *clears throat*) Better still, you can improvise. "Sorry, I don't speak Greek." "Excuse me, was that Greek?" "Does it look like I understand Greek?" "He speaks Greek." "English, please. I didn't take Greek class at school." Sorry, I have nothing against the people of the country. I promise.
“Your answer is enigmatical.” - Benedick, Much Ado About Nothing
If you think that your friend is speaking like Mr. Riddler, just say this. Other possible scenario, maybe in the classroom your teacher doesn't give a satisfying answer. Instead of saying, “Your answer is hard to understand, Sir,” you can say "Your answer, Sir, is enigmatical." 'Enigmatical' is a catchy word, don't you think? If he's an English teacher, he might even give you an extra point for knowing your Shakespeare.
Put them in practice, and if you like, inform me the result. We might enjoy one anecdote or two.