I've just tested and it appears that after you remove someone from your friends list, they don't appear in the search results anymore. I don't know if this is a temporary or a permanent solution, if you manage to scan the code of your friends, then you might be able to add him/her back.
Or you could also try later to see if the username appears in the list. Additionally, if you know the exact username, tap the ghost and click on Add Friends, select to add by username and type the username.