Superted - I think I know the answer to that one..."nothing"...as (I believe) you cannot be an FAA instructor without an I/R.
Also have a feeling - whilst on the subject - that a student only needs an FAA INSTRUCTOR on board if training for an FAA RATING - OR doing any portion of flight that requires the authorisation of an FAA instructor under the Federal Regulations - such as any training before first solo, first solo, first solo cross country, first night solo or first night cross country....and subsequent solo cross country authorisations.
If a UK student came here to do a JAR multi rating or an IMC rating (for example)they would not need to fly with an FAA licenced CFI...because they are not training for an FAA rating, nor flying solo in a type or environment in which they are not already licensed.
(I think)
<<edit for typo>>
[ 09 September 2001: Message edited by: GoneWest ]