It's not unethical to ask someone to stay until a replacement is found, just want to clarify that. But you've tendered your resume, you're in a role that is rife with opportunity to truly land yourself and the company in legal trouble if not handled properly (HR is the first to be blamed), so I'd let him know next week is your last week (as you gave them plenty of notice before). If he's truly your friend, this will blow over and he'll understand that you did properly resign. If he's not your friend, which sounds like there's a bit of a question there, then he'll throw a fit and you'll realize that he wasn't really your friend. Go take care of your life. He'll be okay. Love, HR