If a participant that has been a part of a previous Friends Asking Friends event registered for another event and did not use the Auto Fill link when registering, the previous event's information including Address Book, will not populate into the new event.  Event information is linked based on the username on the participant's contact record. 

Delete the participant's record from the event and have them re-register in the new event using the Auto Fill link with their previous event login. 
Note: The participant will have any associated fees refunded back to them, but will still remain in Sphere.
If the participant is not aware of their previous event's password, you can send them a password reset (How to send a Password Reset for a participant).