The lookup cache can be removed by going into the Address Book and deleting them from the top of the list, one by one. These are removed after a certain period of time (something like 4 hours or so, if that). If it was an email address he typed, you can remove this with Desktop Manager. Go into Backup and Restore then Advanced and look for Quick List (I think); select on the right side and choose Clear. If this doesn't work, then I'll look into it more, but I think that's right.