Do I tell her?

I have been friends with a girl that is now in a relationship with my ex. I don't care if my ex is dating anyone, I have never even considered sabotaging one of his relationships before. I found out yesterday that he asked a girl to hook  up with him while dating my friend. I would want to know if something like this happened to me & my boyfriend, but I don't want to look like a crazy ex. what do I do?