I sold my apple 8Plus, the person who bought it says they need a 5 digit code to open the phone.

Did you complete all of these steps before you sold the iPhone? What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support There is no 5 digit code selection for the iPhone. It is either 4 or 6 digit passcode. If you did not complete the steps, you are going to need to do that. If you did, then there isn't anything you can do for the buyer, except to ask them to return the phone so you can try clearing the device again.

If you completed those steps, and the iPhone is no longer visible when you log into iCloud as a device, then there is nothing for you to do. You can tell the buyer to restore the device to factory and try setting it up again. They may have gotten to the 2Factor Authentication section, and that is up to them to have a trusted device to be sent to, not you.

jusmae1 wrote:

Yes I did. It’s weird, and a little creepy. So what do I do now

Have you suggested recovery mode? Is it still locked to your Apple ID?

You're welcome.