Limit get_next_post to posts from the same author

by

If you add to your WP_Query the author id, next_post and previous_post will automaticly take care of that.

$query = new WP_Query( array( 'author' => 123 ) );
$query = new WP_Query( array( 'author_name' => 'rami' ) );

https://developer.wordpress.org/reference/classes/wp_query/

Hope this helps, stay safe!