I have to see the girl he cheated with?

HI, so a question. My boyfriend cheated on me, the lied about it, and now I have to see the girl he cheated on me with next week. By now she know that he was still seeing me while they started dating, but they are still together.

So i am unsure now how to handle seeing her and talking to her. We do have to work together a bit (he will be there too but i'm just going to ignore him completly). But I don't know yet how to act/react to here. What is your experience with this or how would you handle it?

Updates
+1 y
Note: he's my ex now though, to me he's dead.
I have to see the girl he cheated with?
Post Opinion