I think it's better to allow the tipafriend functionality only for registered users. I had this enabled on a 3.8.x instance, and had to remove it because of spammers. Spammers can use this feature to send unwanted mail without you even knowing it.
See this issue: http://issues.ez.no/8851
I think you can enable/disable this function by means of yout site.ini settting for your siteacces, under [SiteAccessRules]
As for 3.8.x series this function was enabled by default, but perhaps today is the opposite. You would have to change the default settings for this.
The disable setting I did use was
[SiteAccessRules]
Rules[]=access;disable Rules[]=module;content/tipafriend
I think there is a "cpatcha" extension (replicate a string generated by an image) to allow anonymous users send the page URL to other mail addresses. For this see http://ez.no/developer/forum/general/how_avoid_tip_a_friend_abuse
