It rather depends what a court would decide if there was an accident or incident attributable to a lack of supervision.
No doubt the first to react would be the insurers who would withdraw their cover and possibly leave the poor instructor in a position of considerable liability.
The law (Schedule 8) requires that an unrestricted instructor be present at the aerodrome during the take-off and landing at which instruction begins and ends. That's fairly conclusive but refers to the AFI. For the FI(R), it states that instruction is restricted to carrying out under the supervision of a FI approved for this purpose; the latter means that the supervising FI must be nominated to the CAA by the school/club.
As the law is quite specific for the AFI, it is reasonable to assume that a court would expect the same degree of supervision for a FI(R).
You will not get a more definitative answer, as its all down to duty of care, and your responsibility at the end of the day.