Terminate the 10mm end in a IP65 box, then the other end ideally into a metal clad unit, but we dont have one of these. Instead its a Plastic 4 way 63a 30mA rcd unit.
If you're happy to gland into an IP65 plastic box then why all thew fuss about glanding straight into a plastic consumer unit? Seen it done many times, done it many times myself, it's not a problem.
Whether or not you go ahead with this installation, bearing in mind the advice already given, is of course entirely up to you. However, if your friend ever does have any troubles with the council then he may well decide to pass the buck to the electrician that carried out the installation. Then what?