Send mail to user only once when specific check box is selected and updated from profile meta updated by admin

Please add following condition.

$mailSent = get_user_meta($us_id, 'mail_sent', true);
if($mailSent){
    update_user_meta($us_id, 'mail_sent', 'mail sent');
}else{
    $borrower_mail=wp_mail($to, $subject, $body);
}