I am sorry to hear what happened with Matsie. I am so sorry about the dog your friend lost. My sincere sympathies.
My name is Rebecca. I am a veterinarian and will do my best to help.
Is Matsie spayed? Is the other Boxer and the small dog still in the home spayed and neutered?
There is no law saying she has to be put down, but I would be very worried she could attack and kill again. If your friend is going to keep Matsie, I would have her spayed if she is not spayed, and I would not let her around the other dogs at all. She can be out when they are in another room, and if the other dogs are out, she must be confined.
They could train her to wear a head collar like Promise or Gentle Leader, keep it on her at all times, and if she is ever out have a leash attached to the head collar so they can pull her away if she goes after another dog.
It may be she only had a problem with the poor dog she killed but I would never trust her not to kill again. Please let me know what other questions I can answer, and if my answer has been helpful so far. Rebecca