I think that it can really depend on the individual rabbit, on how pellets are going to affect them. I have some adult rabbits that at one time I was free feeding and the only effect was that they gained weight, no GI issues. Then I have other rabbits that if I were to free feed them pellets, they would get sick. I have one baby bunny that I was free feeding like the rest of her siblings, but she kept getting soft poops, so I reduced the pellets and it cleared up, but as soon as I tried increasing the pellet amount with her, the soft poops came back. Then I have that one rabbit that I can't feed pellets to at all because he gets stasis if I do. And some rabbits are going to handle a pellet change better than others, and some rabbits are going to get sick. I think it may also depend on how drastic a change it is from one pellet to another. Like if you are feeding one kind of pellet and change to another brand, but the ingredients are basically the same, then I would think that it would be less disruptive, digestion wise, to the rabbit. As opposed to changing to a feed with different ingredients in it than what the rabbit is currently being fed. I think that in doing a transition, it's not that every rabbit needs that adjustment period, but that you do it just in case you do get a rabbit that would get sick if you didn't transition. And then it might be that some brands of feed cause more problems putting a new rabbit onto it suddenly, than others.
So with your friend, I think it could be over feeding, transitioning to the new feed too quickly, or some other factor like feeding treats or other things that might be causing the bloat. It could be one of these things, or a variety of factors. To be loosing so many rabbits to bloat, would mean that something she is doing is causing the problem. Could also be a problem with her feed. Hopefully she'll be open to suggestions and willing to make some changes to see if it helps her rabbits have less problems.