Well, the secret is out now.
I cannot believe that Peter didn't tell her about Kalinda. It has to be a big blow to Alicia considering the two have been around each other around her since; Kalinda's even been in her home a couple of times. If Peter had told her, I feel that maybe they could work it out, but I fear for the worst now. Alicia probably feels very betrayed knowing that Kalinda sat there with her helping her through Peter's affairs, and come to find out she was one of them. And then imagine Peter knowing what a good friend Alicia considers Kalinda, and to let her go on believing that to be true.
Peter and Alicia's relationship is needed on the show because it is real. It's the picture of a troubled marriage. Many of us have been through it. You find out a boyfriend has cheated and you forgive them, but then you find out down the line that there was more to it than that, like one of the girls was one of your best friends. You've already forgiven them for the cheating, but how do you get over the details you discover later that hit close to home? It's a tough situation that I think will drive a wedge between Peter and Alicia for good.