Idk that I agree with entirely hands off in this situation. Shes in the middle of seeing two people she loves with now knowledge of one of them betraying the other. how can one not do anything about it and watch it happen?

I agree, she cannot tell her mom to stop but she can confront her and let her know that it's no longer a secret. Many times, these things keep going as long as it is successfully done in the dark. Once the light is shed on it, makes it hard to keep going as it has been. AT that point her mother has to make the choice, no one can tell her how to handle it but this could be the catalyst for it being brought to light, hopefully to her father and whatever else needs to follow.

I could not stand by the sidelines watching friends, family, loved ones of any kind betraying another secretly without saying something. In my own mind that's as bad as enabling them to continue and being a contributor to the deception itself.