Hi,
What you can do is configure your checkout to not have the payment directly at the end.
And add the email address of your friend in bcc of the order admin notification so that he gets the email notification too for the sale.
But then, he has to log in the backend and change the status of the order and notify the user. Then, if you configured the order statuses (System>Order statuses) and the "order not payment immediately" setting of the HikaShop configuration correctly, the customer will get the order status notification email with a link for the payment at the end of the email.
It's a bit more complex than just "click a button" for your friend, and this means that you give him access to your backend (you can use ACLs to restrict what he can see but even then, he will be able to see all your orders).