Currently, the AddFriend API allows players to add friends via an email address (the FriendEmail field). If PlayFab finds a master account with the same email address as the FriendEmail, it will add the player as a friend. This works well if the player doesn't mind giving out their login email address, however, if the player would rather keep their login email address private, they should be able to give out their contact email address instead, and have someone add them as a friend using that email address.
TLDR - Make AddFriend work with contact email addresses, in addition to the login email address