two friends are added automatically to each other

I hope that when a user adds a friend, that friend also add, this can be done with the plugin friend request, but is not working properly in my version of Elgg (1.7.5), then I want to add some function of Relationships (but can not find the data model) in add.php in the Friends plugin to automatically add the two users are mutually

What function should I add?

thank you very much

Jp Inteduc