FAA certificates are valid for life. The simplest way is to do training as required, then a FAA Flight Review to validate his FAA Certificate. If you're local drop me a PM if you need me to do a FAA Flight Review. That'll keep him flying to 08th April 2014. After that a validation is required to fly EASA aircraft.
Otherwise it's conversion in accordance with CAP 804, Section 4, Part Q, Subpart 2, Page 3:
(2) The holder of the licence shall comply with the following minimum requirements, for the relevant aircraft category:
(a) pass a written examination in Air Law and Human Performance;
(b) pass the PPL, BPL or SPL skill test, as relevant, in accordance with Part-FCL;
(c) fulfil the requirements for the issue of the relevant class or type rating, in accordance with Subpart H;
(d) hold at least a Class 2 medical certificate, issued in accordance with Part-Medical;
(e) demonstrate that he/she has acquired language proficiency in accordance with FCL.055;
(f) have completed at least 100 hours of flight time as a pilot.
If he hasn't got the 100 hours then he must comply with the Part-FCL requirements in full. However, Article 8 of the Aircrew Regulation states that the hours requirement for ground and flight training is waived if he holds an ICAO licence. (CAP 804, Section 4, Part Q, Subpart 2 Page 1). So it's the same as under JAR-FCL - training as required then written exams and a skill test.
ifitaint...