I dont think his weapons have an anti-friendly-fire system, tho cool interesting concept - we haven't even seen this in more advanced predators in other movies (eg danny glover kills the predator with his own weapon)
she uses the spear when the predator tree jumps and lands by her, missing her with a swipe that imbeds his gauntlet into a tree trunk
naru now uses his spear and he deflects it with his hand-shield but it was on course to hit him, now she goes to use the spear again and it shrinks down - 'locks up' - i assume she inadvertently reduced its size (we know the predator spear has various settings), but after that she stabs him in the leg with it as she wards off his attack
https://www.youtube.com/watch?v=EhwL9ZDnVy0so when his auto-targetting system responds to his projectiles and ends him, there is a logic to it equipment wise
as i've said here, i dont like it, i find it too much of a stretch to imbue her with such an expansive intellect, even tho she has shown herself to be smart and a quick study and we saw her witness the fact it is the helmet that targets his projectiles
now Prey is no more guility here than many films/tv where we see humans (or others) quickly and sometimes instantly use completely alien or extremely advanced systems/technologies that they've no right in having a clue about, but still, i wished they'd gone a different way with the killing blow