Apple watch reset

I have a relative who spent a long time in hospital (many months). He had an apple watch which was linked to his old phone.

He then replaced his phone (but didn’t unpair/detach the apple-watch from this old phone before factory resetting and selling it).

He has tried resetting the apple watch and linking to his phone/apple account but he is struggling. Instructions seem to take us into a loop cycle which includes needing his old phone.

Is there a clear, easy way of resetting the watch and linking back with his apple account but not his old phone (there is no data on the watch that he needs)? He is out of hospital now and the watch might help him recuperate and provide a motivational boost.

Thank you very much for your help.