Any FI or FI(R) may conduct differences training if they are qualified for the difference themselves. An FI(R) would of course be supervised by another FI.
By signing your logbook to say that you have completed the training, they are in effect placing their neck on the line, so it is in their interest to ensure that you are competent.
JAR-FCL assumes that FIs are professional people who do their job properly, it should not be necessary to tell them how to do it!