Watches do not automatically pair to iPhones, You need to pair it manually.
But that would mean it would no longer be paired to your iPhone, and you would not get your own calls and messages on it, but your friend's from his iPhone instead.
Also all exercise data would sync to his phone, and you would get his Watch Data and Apps and erase yours from the watch.
Do not pair your watch to anyone else's iPhone. There is no real point to doing that.