I agree that the earth borrowed from the RFC is probably quite adequate to provide the necessary protection to the cooker circuit. However I would not accept the practice as an actual solution specifically for two of the reasons already mentioned:
a) The original cooker cable is either damaged to an unknown extent, or connected in a defective manner. It is impossible to say what other danger exists from this problem, but borrowing the CPC conceals the original defect and is likely to prejudice its timely repair. This is a different scenario to borrowing a CPC for a circuit that was installed without one, where nothing is otherwise defective.
b) In a domestic situation, confusion is likely to arise from the non-standard configuration, especially as one of the circuits involved is an RFC, and erroneous future test results and inferences from them can reasonably be anticipated.
This latter point does not in my mind stand comparison to a metallic containment providing the CPC for multiple circuits, which is a normal and expected configuration, nor the deliberate design of a wiring system with one CPC cable serving multiple circuits. This occurs quite commonly in entertainment lighting, where cabling often runs in blocks of 6, 12, or 24 identical circuits to a single point of use. There's no point running 24 CPCs in parallel to the same box, one cable will do, but it is plainly obvious which CPC serves which circuits even without documentation and the circuits can be tested without unexpected side-effects from cross-connections.
By contrast, this ad-hoc arrangement is anything but obvious. Part of the cooker circuit cable is protected by the usual CPC from the CU, the other part terminating in an unknown location, and neither properly connected to, nor insulated from, the first part, is back-fed along with the cooker itself, from a CPC consisting of two legs of a functionally unrelated RFC.
All that said, I might be tempted to do this as a fix but I'd make it visible and ugly, so that the problem wouldn't get swept under a rug!