I will admit that it sounds odd. 9 week old babies can be starting to get hormonal, but it would be quite rare for them to be aggressive to the point of killing a litter mate for no apparent reason. I could see 9 month old rabbits doing it, but 9 weeks just seems odd. If it did happen suddenly when they were being fed, it stands to reason that someone was there. So not trying to do something is also odd. I know rabbits can do a lot of damage quickly, but just letting them go at it, have to wonder about someone who would allow it to happen.
I would not call 9 week old babies bonded, as babies tend to get along anyway, but they would not really be at risk of hormonal issues yet.
If the person is wanting some refund, I would question it. I don't know what sort of guarantee the breeder offers, but this situation doens't seem like something that would have been the breeders fault (unlike maybe a health problem that would have been there when they were sold). A necrospy could be done to determine the actual cause of death and if there was anything else going on.