Disable visual editor on one specific page

In your code, calling the action admin_init makes is_admin() unnecessary. And, if not mistaken, is_page() is meant to be used in the front-end…

But the solution is the following (based on this Answer):

add_filter( 'user_can_richedit', 'wpse_58501_page_can_richedit' );

function wpse_58501_page_can_richedit( $can ) 
{
    global $post;

    if ( 28 == $post->ID )
        return false;

    return $can;
}