I think it quite possible N2 kept running.
In the CF6 the N2 mainshaft bearings are independent of the N1 shaft, and in development testing of a new core the factory might run a "core only" test. The fan & LPT (N1 system) is not a requisite for this kind of test.
With no LPT to provide back pressure on the core, the N2 will still run at its governed value, but all air pressures will be very low - thus not much fuel is required or used. The fuel control N2 governor will take care of that. That's why I say that N2 could have kept running - until voluntary shutdown, or oil starvation, anyway.
Of course, if the LPT departure damaged the aft core bearing, then N2 probably seized up quickly.