BCSE points referral mod does this
http://www.bcsengineering.com/store/refer-a-friend-customer-reward_points-add-on.html
but it's an add-on to the points system -- so it's not as simple as you described.
I have this for our points system but unfortunately, very few customers take advantage of it. But what's very interesting is that many or even most of the customers who receive the coupon take advantage of it if their friend sent it to them, so this can be a very effective way to have friends bring friends to your store.
I guess I need to find a better way to beg our customers to refer their friends?
