Set the default value for private messaging option on user register or add.

private-message-default.php

<?php

/**
 * Set the default value for private messaging option on user register or add.
 * @param int $user_id The ID of the newly registered user.
 * @return void
 */
function um_change_who_can_pm_default( $user_id ) {
    // Set the default value for private messaging option to 'friends' for the new user.
    update_user_meta( $user_id, '_pm_who_can', 'friends' );
}

add_action( 'user_register', 'um_change_who_can_pm_default', 10, 1 );