Pass a Skill Test with an examiner lasting about an hour. If he hasn't flown for 5 years he may need to do some training first. If the aircraft rating has expired by more than 5 years it has to go to the CAA for renewal; less than 5 years and the examiner can renew it.
He then needs to look at the licence to see if the licence itself needs to be re-issued i.e. is it a 5 year licence or a non expirying Nattional licence. If it has to be reissued, then he needs a valid medical certificate and an aircraft rating that was valid in the last 5 years.