Activation Lock is designed to prevent anyone else from using an Apple Watch if it's ever lost or stolen.
About Activation Lock on your Apple Watch - Apple Support
Activation Lock is an anti-theft feature that cannot be bypassed. Only the previous owner can remove the lock by using their Apple ID & password. Apple will not help you find the previous owner. If the previous owner cannot or will not remove the lock, then the watch is useless to you or anyone else. In the future, see the 'Before you buy a watch from a third-party' section in:
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
If your friend is the ORIGINAL owner, then he should see the 'Start a support request' section in:
How to remove Activation Lock - Apple Support
Also see lobsterghost1's tip:
The All Too Common SAD Reality of Buying … - Apple Community
If it needs a new passcode (instead of an Apple ID password) then see:
If you forgot your Apple Watch passcode - Apple Support
Be sure to put the watch on its charger and keep it there until you complete those steps.