Good solid advice, @juillet
This boils down to one question - if Penn made an offer that included no financial aid whatsoever, and MIT was off the table, would the OP attend Penn. If the answer is yes, the Penn is not affordable, and claiming them are once an acceptance to MIT appears is unethical. It really doesn’t matter that the student is only 17, and doesn’t understand contracts or family finances. His parents and Guidance Counselor also have to agree to this tactic.
If MIT were to send their decision out first, and the OP were to withdraw from consideration at PENN, that would be a different story. But that’s not going to happen. Contract law requires both sides to give some form of consideration. Penn is offering an early decision - a commitment on their part. The Students who apply ED are offering their commitment as well. If they are not sure they can afford to attend, and there are little to no questions about possible aid, they should be able to make the decision the day they get their offer. I can see this being an issue for someone who has a unique situation where the financial aid offered doesn’t match the NPC. But if the NPC already indicates full-pay, that won’t be the case. Sticker-shock families are going to know right away that they can’t afford the school, and back out immediately (and are most likely going to have to start a new list of affordable schools).