Are you going to have a Tesla in the future? If not, then I wouldn't expect it to be a big deal, because you wouldn't expect any conflicts from Tesla. But it's your account not his, so it's your name having accepted the TOS for using the car and the services like Supercharging.
If you are going to have any other Tesla vehicles, this would be bad. But if you are leaving it all behind I could see doing it as a favor to the new buyer.
It's a really nice perc, but consider negative scenarios, like the new guy abusing the hell out of free miles, like starting a limo service or driving for Uber on free miles. That's now on your account/name.