users#index and users#show renders additional field (relation_to_current_user) which holds information about relation of rendered model towards current user.
Values:
SENT_FRIENDSHIP_INVITE - user sent invite to current user
RECEIVED_FRIENDSHIP_INVITE - user received invite from current user
FRIEND - user is a friend to current user
STRANGER - users are not friends and they don't have any pending invites
To check if user has already invited me to friends or if I invited him I must get all friendship invites relative to both users.