Did he tell you he was going to sleep with her BEFORE he did? If not,then even though you were on a short break,he should have been upfront about his intentions. I know my primary is still sleeping with his ex,he told me and initially I found it difficult because she has said some nasty things about me. However once we discussed it,he made it clear that he won't let her wreck what we have.
Rick is right,some boundaries need to be worked out with all 3 of you because your expectations of this relationship may be different to both hers and his and in order for it to work,everything needs to be laid out in the open. It is possible(though not absolute) she may have an ulterior motive so it would be wise to keep your ears and gut open to subtle changes in the dynamic.