Well, since you appear to be trolling for an argument, I'll bite.
The demon player should encourage the player with the force weapon to take the psychic test if at all possible.

After all, it's a 2 in 36 chance of causing a difficult to avoid wound on that model, and everything in that book is immune to instant death, anyway.
Whether or not that happens, it's a force weapon. You don't get to make it stop being a force weapon, so if it causes a wound, that demon gets a 2+ save.
I don't see any possible argument about Kharne's ability, which is completely different despite having an identical name.