I up-dated the 'Tell A Friend and Shipping is Free' module.
I fixed some bugs, added admin control for editing, deleting and mailing referrals. Also added conditions so it's ether on at all times, or after one of the following,
=> cart total
<= cart total
=> total Items
<= total Items
I haven't installed this on a live site yet, ran on a testing server. As with any add on, please test it fully before adding to a live site
You can find it here Tell A Friend