I believe that Aesir is right.
It is possible, according to JAR, for an FI(H) to instruct on single pilot multi-engine helicopters, so long as he/she complies with all the bits in JAR-FCL 2.330 (f). In the same way, it is possible for a FI(H) to do the work of an IRI to do instrument rating training too, so long as he complies with the relevant requirements in 2.330.