Most Helpful Girl
You could be "mature" and give her what she doesn't deserve: your time and energy by even acknowledging her and being friendly.
Or you could literately treat her like air for sh*tting all over the bond you worked so hard to build. Act like she is no more significant than the dog sh*t you passed on the street, making sure not to step near it. It will make her feel small and that's what she deserves.
I'd opt for the second alternative. Cheating is disgusting. That's one of the few things I'll never openly forgive someone for. I'll forgive them in my heart so that I can have peace of mind in my future, but I'll never openly forgive them so that they can forgive themselves because that regret will eat them alive and that's what they deserve.
Most Helpful Guy
Just pretend she doesn't exist. Go with the idea you don't know her anymore. So when she does say or something just say, "Who are you?" Childish? yes, but you'll feel a lot better than simply ignoring her.