Q: Is it possible that on apple watch are unknown sms?

Hi

Some other users have reported seeing messages on their Apple Watch that are not also on their iPhone.

For example:

Anyone else seeing blank text message on their watch?

imessages not showing up on iphone

I appreciate that - please also read the second linked thread.

If your friend is concerned that their Apple ID may have been compromised, they should reset their Apple ID password (they will need to sign in again on all devices):

Change your Apple ID password - Apple Support

Having done so, they may also wish to unpair their watch and then re-pair it to their iPhone:

Unpair your Apple Watch and iPhone - Apple Support

Set up your Apple Watch - Apple Support