Disable messages for non friends.

disable-messages-for-nonfriends.php

<?php

/**
 * # Disable Messages for non friends.
 */
function yzc_disable_messages_for_nonfriends( $html ) {

    if ( ! friends_check_friendship( bp_loggedin_user_id(), youzify_get_context_user_id() ) ) {
        return false;
    }

    return $html;
}

add_filter('yz_get_send_private_message_button', 'yzc_disable_messages_for_nonfriends' );