Hi!
It is
extremely rare that piggies kill and they don't usually do it intentionally unless as a very last resort or as an accident during a bad confrontation where they cannot get away from each other. They won't do so unless seriously stressed out.
It doesn't usually happen in well cared for guinea pigs with plenty of space to get away from each other; the cases of deadly bites we have come across have been less than a handful and only a couple of these cases has happened in a pet and not in a breeder environment; both cases were of badly fighting teenage boars where deep bite had accidentally hit a vital spot.
Instinctive deep defence bites only happen in very riled up piggies that are totally on edge, and they will hit the first thing that moves with full force, whether it is your hand or another piggy getting accidentally in the line of fire. Lack of space and therefore an escape route that would avoid serious confrontations is very often at the very heart of all the tragedies.
Sadly these kinds of incidents are much more common in breeder circles where there is generally less attention to natural guinea pig instincts and behaviours and often much less space provided so that overall stress levels are higher. I can usually tell which of my adoptees have originally come from a breeding background.
My suspicion is that your friend has likely a) not provided enough space, b) not taken any respect of the fact that sows whose relationship was already strained before they had babies may not go back together afterwards and and c) refused to separate after it must have become clear pretty quickly that the two sows would not get on well anymore. Sadly, it has come at the cost of an innocent life.
We have seen several fall-outs on here between sow pairs with an unplanned pet shop pregnancy when the undersow was the mother but refused to go back into that position now that she herself was leader of a new group. You don't force them back together and rather keep them separate with their babies or with a new companion that cannot challenge their leadership. Adult sows won't generally happily submit to another one once they have drunk from the heady cup of First Ladies - as little as stud boars will like being stuck back with another boar once they have been with a sow as their life career aim is to be chosen by a group of sows as their companion and father of their babies.
" Biting" And What You Can Do (Biting, Tweaking, Nibbling and Nipping)