When Friends Fight 
Dogs that normally get along, then seem to fight for no reason, can be a tough puzzle to solve. I agree with other posters make sure everyone is healthy.  It is ok to let the aggressor know what they are doing, is not allowed. Use a firm voice and use the same  cue word like Stop It, or No Way, every time  The idea is to redirect the dog. You can clap, stomp a foot, toss a pillow at her or give her a time out alone in another room.  It will help to read some YT threads about dogs fighting. I would also suggest starting the 
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