I think the king is not the best friend, I think he is Will's father, I mean, that best friend of his is the same age as his, and we clearly saw Will was the first one to be reincarnated, It's impossible for the friend to suddenly be older. We also need to remember the fact that his father dissapeared not dead, also remember the part where he thought that the king's voice was familiar? I really think that the King is Will's father from where he came from
Tensei Shichattayo (iya, gomen)