If the Apple Watch is still listed under the registered devices for the previous owner, she can also disable the Activation Lock on the web:
...
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:
- Go to iCloud.com and sign in with your Apple ID.
- From the Quick Access Menu in the top right, select Find My. You might be required to sign in.
- Select All Devices, then click your Apple Watch.
- Click Erase Apple Watch. Select Next until the device is erased.
- Click the remove button
next to your Apple Watch.
...
copied from: About Activation Lock on your Apple Watch - Apple Support
The official Apple Support can only disable the Activation Lock, if you have proof of ownership, as explained in this article:
Additional Support - Activation Lock