No he does not need an additional 20 hours of instruction from a FI authorised under JAR-FCL. He needs to have achieved a total of 20 such hours, so whatever instruction he received from a JAR-FCL FI towards his NPPL will count. For example, if your colleague had obtained his SSEA Class Rating by conversion from some other licence and had received only a handful of hours with a JAR-FCL FI, then he would need to make up the difference before he could be issued with a JAR-FCL PPL(A) with SEP Class Rating.
Rating to an existing JAR-FCL PPL(A) requires the training and testing you refer to; however, if he intends to open a JAR-FCL PPL(A) with MEP Class Rating without having first upgraded from NPPL with SSEA Class Rating, he would probably have to meet all the NPPL-to-PPL upgrade requirements on an MEP Class aeroplane - he would need to speak to the Authority to confirm this.
Simplest process is probably to meet the upgrade requirements for NPPL with SSEA Class Rating to JAR-FCL PPL(A) with SEP Class Rating first, then complete the MEP Class Rating training, then apply for JAR-FCL PPL(A) with both SEP Class Rating and MEP Class Rating - unless, that is, the Authority require him to be physically in possession of a JAR-FCL PPL(A) with SEP Class Rating before he can add the MEP Class Rating.