I fail to see any kind of moral dilemma here what-so-ever. She is aware of a woman doing something, which more-or-less amounts to murder. It is absolutely her responsibility to notify the police.
Of course the police can't just arrest and charge a woman soley on the basis that so-and-so made an allegation against her. They will have to gather evidece and build a case against her. But this is no different from any other crime.
As for lacking the courage to stand up in court - I'd imagine that there is some way that she can notify the police anonomously, if she really feels that that is neccessary.
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