Runner on 1st ball hit to 1st baseman and she goes to tag Batter/Runner going to first, runner stops and takes one step backward. She was called out as soon as she stepped backwards. This part I agree with 100%. The part that i'm unsure of is that the ump awarded R1 second base, ump said she was already at second before B/R stepped backward.
I believe R1 should have to return to 1st base because B/R is not allowed to go backward, she is allowed to stop but not retreat.