"Tomorrow, Duo visits his friend, the king."
Translation:Morgen besucht Duo seinen Freund, den König.
That's not how we say it in German.
The time specification Morgen is part of the sentence (not separated from it by a comma), takes up the first position if you want to put it first and so the verb has to follow it immediately in order to be in the second position where it belongs.
can you not say "seinen freund, DER König" because der König is like a separate sentence meaning who is the king?
No, it is an apposition and has to be in the same case as the word it refers to,