Used Apple Watch is not accepting my Apple ID and password

your friend needs go to iCloud and turn off the Activation lock

Turn off Activation Lock before you send your Apple Watch in for service, sell it, or give it away

Before you send your Apple Watch in for service, sell it, or give it away, you should turn off Activation Lock on your Apple Watch. Just unpair your Apple Watch from your iPhone:

1. Keep your Apple Watch and iPhone close together. 
2. Open the Watch app on your iPhone.
3. Tap the My Watch tab, then tap All Watches at the top of the screen. 
4. Tap the info button  next to your Apple Watch.
5. Tap Unpair Apple Watch. For cellular models of Apple Watch, tap Remove [Carrier] Plan. If you aren't going to pair with a different Apple Watch or iPhone, you might need to contact your carrier to cancel your cellular subscription.* 
6. Enter your Apple ID password. If you forgot your Apple ID password, you can reset it. 
7. Tap again to confirm. 

If you can't access your iPhone or Apple Watch, or if your Apple Watch isn't paired with your iPhone and you need to turn off Activation Lock:

1. On a computer, go to iCloud.com and sign in with your Apple ID. 
2. Go to Find My iPhone.
3. Select All Devices, then click your Apple Watch.
4. Click Erase Apple Watch. Select Next until the device is erased.
5. Click the remove button  next to your Apple Watch.

* Carrier fees may apply. Contact your carrier for details. 

https://support.apple.com/en-us/HT205009