That's absolutely correct, the instrument proficiency check must be conducted in an aircraft that is appropriate to the aircraft category, which in your case means it must be done in a helicopter or heli-sim.
Now, I'm not sure if it would be possible to do the IPC in a helicopter or heli-sim with a fixedwing CFII, because the regs only say:
PART 61.57 (d)
(2) The instrument proficiency check must be given by—
(i) An examiner or,
(iv) An authorized instructor
I know for sure that an IPC can be done with a CFII, because that's how I regained my IFR currency in the past! So I can't see why it should not be possible for you to do your IPC with a fixedwing CFII in a heli-sim?!