I think perhaps that he is after the JAA pilot to be PIC and him to sit there logging safety pilot time under the FAA rules. Either way he plans it as a JAA pilot should the CAA ever query my hours I would not want to be in the position where I am asked pointed questions about how I got my hours. Worse still is if he was later to convert to JAA and the CAA find out that we had both logged time on the same aircraft on the same date, time etc. Better staying well clear if you are JAA of these types of arrangements IMHO.