There is no reason why you should not revalidate his rating, that is what JAA is all about. However, it does pose one question: which form do you use; and who do you send it to? In theory we should all have the same form, but we don't! When you send the UK form to the Swiss CAA will they recognise it?
An interesting point is that the Swiss don't accept UK National licences, only JAR-FCL Licences and ICAO licences issued by non JAA States.
Which licence does your candidate hold? A JAR-FCL or a Swiss National Licence, whilst in theory you could revalidate either, it could be a one way deal!