We know the LM2577 cannot directly drive the forward converter primary because of the voltage so the outboard MOSFET must be tied into that task somehow.
It is a shame that the design allows a shorted out electrolytic filter capacitor to take out that many components. There is no intrinsic current limit built into this design. Had there been, it is likely it would have survived with just the replacement of the filter cap (which Lee did replace, but it was already damaged at that point). The current limit for this design is a pair of fuses on the final output, well downstream of this shorted capacitor.