Perhaps the mistake will be caught before they actually refund the money. But if it isn't, and they refund it (presumably to a credit or debit card), I don't think you would "get in trouble" since this was Best Buy's mistake and not yours.
The morally correct thing to do would be to contact them and explain that you didn't return the item and that they would be refunding the money to the wrong person.
One downside of doing nothing, is theoretically if they discover the mistake sometime in the future they may have the legal right to charge you again, and that could be an inconvenience if you aren't prepared for it. That being said, just because they could do that, I don't know that they would. If they did I hope they'd at least contact you first.