Please ask your friend to look at this Apple Support document,
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
Particularly the section about deleting the watch from her iCloud account and Apple ID using a web browser. I am pretty sure her iCloud and Apple ID still exist, because you are being blocked. Most likely she still has her iPhone and still using the same Apple ID because they never go away even if you delete all the linked devices.