First off, I am so sorry to hear what happened. I would be beside myself if one of my dogs killed one of the cats. I have been very fortunate that mine all get along.
That said, let's see if we can't try to make some sense of this whole situation.
Kitty & Boxer never had issues. Okay, great. BUT - did they play/interact with each other, or did kitty keep its distance and Boxer only get to see it from across a room? Did kitty initiate play, only to back off and try to retreat when Boxer got too carried away? These things when supervised would be somewhere that a human may need to intervene, but if left home alone, might turn into an accidental injury/death.
ANY dog has a prey drive. Some breeds are just more in control of them on the whole, and in some breeds where it is more of an issue (such as huskies or greyhounds - yes, greyhounds) there are some individual dogs able to suppress the urges better than others. It varies from dog to dog.
A Boxer is a dog, therefor has a prey drive. Boxers are not always good with cats. If you look at a few rescue sites, you will see many Boxers listed as "No Cats" or "Didn't get along with cats in home". My personal experience is that the dogs are far too rough and the cats get overpowered....but I am sure this is not the case 100% of the time.
Boxers play rough. We all know this.....

. They seem almost unaware of their size and strength, and those big ol' feet can get them in hot water unintentionally. And when you have a pup (2 is pretty close to the high end of puppydom), their clumsiness and flailing feet tend to be even more out of control. Is it possible that the 2 got to playing and the Boxer got carried away?
Your friend needs time to grieve, and I am glad that you were there to step in and take the dog in while she does this. She also needs to consider that she cannot blame the dog forever. The dog has, I am sure, forgotten the incident by now, and in fact may even seek out kitty as if nothing ever happened if/when it returns home.
Hopefully, your friend can see the situation for what it was, what it could have been, and not banish the dog if it was something which could very well have been nothing more than exuberant play that got taken too far.