If you can, lose the formality.

By definition, the familiar terms are, well, more familiar. Whereas formality is a constant reminder of rank. I for one felt the constant reference to Frodo as master by Samwise was jarring and anachronistic (to me at least.)

It won't be jarring with royalty, like it is for me with the hobbitses. It works for Horatio and Hamlet. But the most important thing to consider with that formality is that it is a constant reminder to all characters and the reader alike of rank.

So my question to you will then be will the prince ever pull rank? If he is the type of person that is wont to do so then he will probably insist on the formality as a reminder. Will the friend follow a direct order unwaveringly? Even if it is against their character, moral compass, idealogical mindset, or best interest? If so they will almost implicitly call the prince by title as a sign of duty and respect. Or will the friend almost certainly follow their own path if they were forced to choose between the prince and their own personal guidelines? If so, I think they are not likely to use titles with their friends, unless held to do so by the sword (which would almost certainly end the friendship.)

I say it is ultimately a matter of the characters themselves betting their personalities, however, I also suggested you choose one over the other unless you had to. The reason I said that is because formality will always be a small sliver of a wedge in any friendship. As a reader I will always be reminded that one of these people is more important than the other. And they both know it.

If you can, lose the formality.

By definition, the familiar terms are, well, more familiar. Whereas formality is a constant reminder of rank. I for one felt the constant reference to Frodo as master by Samwise was jarring and anachronistic (to me at least.)

It won't be jarring with royalty, like it is for me with the hobbitses. It works for Horatio and Hamlet. But the most important thing to consider with that formality is that it is a constant reminder to all characters and the reader alike of rank.

So my questions to you are: Will the prince ever pull rank? If he is the type of person that is wont to do so then he will probably insist on the formality as a reminder. Will the friend follow a direct order unwaveringly? Even if it is against their character, moral compass, idealogical mindset, or best interest? If so they will almost implicitly call the prince by title as a sign of duty and respect. Or will the friend almost certainly follow their own path if they were forced to choose between the prince and their own personal guidelines? If so, I think they are not likely to use titles with their friends, unless held to do so by the sword (which would almost certainly end the friendship.)

I say it is ultimately a matter of the characters themselves letting their personalities show, however, I also suggested you choose one over the other unless you had to. The reason I said that is because formality will always be a small sliver of a wedge in any friendship. As a reader I will always be reminded that one of these people is more important than the other. And they both will too.