I dont know the answer but .. .. ..
logically I would think it to be no - unfortunately .. .. ..
because if he has lost his medically presumably they feel he could have a medical "event" in flight which could incapacitate him. If this happened you would no longer be PIC under training.
Not wishing to be pedantic but presumably you mean he has an FAA IR not an IFR. Presumably the aircraft is on the N reg?