Well, you are both correct...and both wrong.
Exhibit A: Closing In
All of them were firing as the Shivans closed in...HOWEVER, it is also worth noting that by the time the female trooper in question fires, the muzzle of her weapon is not in view and so muzzle flashes
cannot be observed. This leaves us with the body animation, which suggests she fired a weapon with heavy recoil twice.
Exhibit B: Who The Hell Is That?
Shot is almost certainly of the center, male trooper. The gun is wrong for it to be the female trooper with the weapon in question, and the view is wrong for any other besides the center man as viewed from behind and slightly to the left (interestingly, roughly the posistion where there was another trooper).
Also please note apparent spot illumination
from the right side of the Shivan's leg, probably from another Terran's weapon-mounted light. The only trooper to the right is the female trooper with the weapon in question. This is reasonable proof she was pointing her weapon in the direction of the Shivan which got whacked.
Exhibit C: Shivan Going Down
While the man is in fact moving back, there is no muzzle flash to indicate he is firing his weapon as either AR or grenade launcher. Partial lighting of the Shivan's "head" can be accounted for by the fact the Terran troopers had flashlights affixed to their guns; angle for such illumination is roughly correct. The trooper illuminating it would be, based on rough shape, size, and apparent angle of the illumination, the female trooper with the weapon in question. This operates both for and against the theory she fired: her weapon was pointed at the target, but recoil may or may not have caused it to move off-target. No good shots are available of her firing her weapon to show in which direction it largely recoils.
Conclusion: while impossible to completely disprove he did not fire a grenade or some kind of AP projectile himself, it seems highly unlikely based on the lack of apparent muzzle flash. It is also worth noting that last shot of the female trooper with weapon in question had her lit by muzzle flash, presumably from her weapon.
