To answer your first query, we don't delete the user data from the backend also we don’t purge it. It doesn't matter how old your account is, we don't touch user data. Please make sure that you have not deleted the user by your own from delete option in AppHQ console.
To answer your second query, both of services are independent to each other, so if you delete the user from user service, it will not going to delete the existing details of that user from App42 data base. That's why you are able to find that user in GetAllFriends method.
Please check & let us know if you have more question for us.