As NJ states, an FPN is not the same as a criminal conviction. The Home Office website confirms this:
http://www.homeoffice.gov.uk/anti-social-behav iour/penalties/penalty-notices/
(See the final paragraph).
Since there is no criminal conviction, the Rehabilitation of Offenders Act does not apply. (i.e. there's no defined period for an FPN to become 'spent').
If the person in your question is asked, on an employment application form (or at a job interview) "Do you have any criminal convictions?" or "Do you have a criminal record?", that person can (assuming that he/she has no actual unspent convictions) lawfully answer 'No'
If an employer carries out a CRB check at 'standard' level, an FPN will not show up.
If an employer carries out a CRB check at 'enhanced' level, the FPN will not
automatically show up, but a senior police officer can include any information which he (or she) deems to be relevant for the purposes of the check. Such information can include FPN's, unsubstantiated allegations against the job applicant or even information about the applicant's partner or family members.
Chris