[geckoday]

I'd go about it this way:

skin1/customer/main/product.tpl
Modify to pop up window url product.php?productid=xxx&mode=showsend when button is clicked for send to friend
remove include of send_to_friend.tpl

send_to_friend.php
Modify to set $main="showstf" and assign to smarty variable if mode=showsend. See first few lines of product.php for how its done.

skin1/customer/home_main.tpl
modify to use customer/main/send_to_friend.tpl when main=showstf

skin1/customer/main/send_to_friend.tpl
modify as desired for your popup window

This way all variables that are available on your product page are available on your send to friend popup page and you use the normal X-Cart sned to friend logic to send the email.