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My friend had two Roseys that she was told were a pair. Well, quite a bit of time has passed and they did nothing. So, today, she seperated them and put what she thought was another female in with what she believed was a male. She came back an hour later and the original bird had nearly killed the one she just put in with it. I am really concerned, because the bird that did this damage is one she is giving me on a trade. I have never heard of this level of aggression between Roseys. She is going to put the other one back in with its original cagemate...and that is the pair I will be getting. I have four other Roseys. I am not sure if I should add these two of hers to my flock. Any feedback ?

Not a risk I would take, and I surely wouldn't want to pass that temperament on to chicks.

That is very strange. I have probably bred thousands of Bourkes over the years and never had this happen.

However, really she should have kept the new bird and "old" bird in two cages side by side for a few weeks and then introduce both of them to a new flight, so neither bird has established territory. (though Bourkes are not territorial).
You don't state how big the cage was, maybe it was too small and the established bird had bonded with the other bird and felt "crowded" by the new one.
Also, I would take the birds, it wouldn't bother me. I would make sure to introduce him slowly to any new bird, make sure tht he is introduced to the females cage (not vice versa) and keep his wings slightly clipped.