Hi,
Do to so it requires HikaShop business in order to have "item" and "order" custom fields.
Thanks to that, you can create a custom field (item or order) and use the HikaSerial plugin "mail to custom field".
The customer will have the input to enter the email address of his friend (in the product page if you're using an item custom field or in the checkout if you're using an order custom field).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.